Theme: "STUCK IN THE MIDDLE WITH YOU" - U is inserted in the very middle of each common phrase.
27A. Complete text of "Prefixes for Dummies"?: THAT'S A NOUN STARTER. That's a non-starter.
40A. Fancy hotel rooms overrun with spiders?: WEB SUITES. Web sites.
65A. Negotiating with Mephistopheles?: ON THE FAUST TRACK. On the fast track.
78A. Tariff on marble sculptures?: STATUE TAX. State tax.
110A. Snap selfies in a sulky pose?: TAKE POUT SHOTS. Take potshots.
71D. Candlelit dinner at the Geek Squad counter?: BEST BUY DATE. Best by date.
74D. Sotheby's event featuring a Shakespeare original?: PLAY AUCTION. Play action.
97D. #marinara, e.g.?: RAGU TAG. Ragtag.
This is how constructors elevate a common letter addition/deletion/replacement. Katie and Rich insert a U exactly in the middle of each familiar phrase, adding an extra layer of precision.
As a result, all the theme entries have an odd number of letters. We also get a rare left-right mirror grid. Lovely gridwork.
Across:
1. Fill one's ride: TANK UP.
7. Telly channel: BBC.
10. In fine form: FIT.
13. Invent: CREATE.
19. Julia's "Seinfeld" role: ELAINE. Elaine Benes.
20. Notable period: ERA.
21. Messenger __: RNA.
22. "Memo" on a memo, maybe: HEADER.
23. Cosmetics sample: TESTER. These little testers leak easily.
24. Give adequate reason for: WARRANT.
26. Pick up, as a call: ANSWER.
30. Six-time Oscar nominee Adams: AMY.
32. Avenger with a hammer: THOR.
33. Mixed martial arts org.: UFC. The Ultimate Fighting Championship.
34. Young fellows: LADS.
35. Texter's "I think": IMO.
38. Make like whales and icebergs: CALVE. Calve also means "to break off a large chunk". Learning moment for me.
43. Tennessee NFLer: TITAN.
45. Some children's party performers: CLOWNS.
47. Argentine soccer star Lionel: MESSI. With Inter Miami now.
48. Wild cards, maybe: JOKERS.
49. Some antique cars: REOS.
50. Grub: CHOW.
53. Valley where it's easy to get a cab?: NAPA. Cabernet.
56. Part of the HOMES mnemonic: ERIE.
57. Inquire: ASK.
58. Satisfactory: GOOD.
59. Taxpayer's rep: CPA.
62. Nike rival: PUMA.
64. Took a chair: SAT.
69. Trembling violin tone: VIBRATO.
72. Scotts Miracle-Gro brand: ORTHO.
73. Orbital shape: ELLIPSE.
77. Build: ERECT.
81. Place for a trim: SALON.
82. "Don't __ me!": SASS.
83. Passes, as time: ELAPSES.
84. "The Sweetest Taboo" singer: SADE. Sweet, soothing voice.
85. Stone fruit's stone: PIT.
86. Some Italian sports cars, familiarly: ALFAS.
89. Glee: MIRTH.
93. "That hurts!": YOW.
94. Thumbnails in a music collection: ALBUM ART.
96. "The Mole" host Shapiro: ARI.
99. Pup's front feet: FOREPAWS.
101. Surgical stitch: SUTURE.
102. Sought office: RAN.
103. "A __ of Their Own": LEAGUE.
104. "Can't deny it!": YES I AM.
106. Booker T.'s band: MGS.
107. Pacific Northwest hub, familiarly: SEA-TAC. Seattle–Tacoma
108. "Deets coming soon": TBD.
113. Airport safety gp.: TSA.
116. Esquire publisher: HEARST.
118. Largest of the British Virgin Islands: TORTOLA. Lots of offshore companies there.
119. More spooky: EERIER.
121. Laces up again: RE-TIES.
122. Floor covering: AREA RUG.
123. First words in a journal, maybe: DAY ONE.
124. Offer of assistance: USE ME.
125. Financial records: LEDGERS.
126. Gives for a while: LENDS.
Down:
1. Lunar holiday in Vietnam: TET.
2. Drink served in a yard?: ALE. Yard of ale. Yard-tall beer glass.
3. Angry glare: NASTY LOOK. And 16. TV spot scribes: AD WRITERS. Symmetrical partners.
4. __ and kin: KITH.
5. Like leftovers: UNEATEN.
6. Western Australia city: PERTH. Capital of Western Australia
7. Word of caution: BEWARE.
8. Muffin fiber: BRAN.
9. Go on a bender: CAROUSE.
10. Saint of Assisi: FRANCIS.
11. Travelers' stopovers: INNS.
12. Tell (on): TATTLE.
13. Leafy green in the beet family: CHARD.
14. Signs a lease with: RENTS TO.
15. "At __, soldier!": EASE.
17. Golf ball prop: TEE.
18. Drop the ball: ERR.
25. Singer-songwriter Wainwright: RUFUS. Unknown to me.
28. Plant, as seeds: SOW.
29. Small batteries: AAS.
30. Capital of Ghana: ACCRA.
31. Drones, e.g.: MALES. Bees.
36. "The Sound of Music" governess: MARIA.
37. Get-go: ONSET.
39. Bugs on the road, briefly: VWS.
41. Automaker based in Munich: BMW.
42. Element with the shortest name: TIN.
44. 34th prez: IKE.
46. Joplin known as the "King of Ragtime": SCOTT.
48. Denver Nuggets point guard Murray: JAMAL. Canadian.
51. Hostess goodies: HO HOS.
52. Celebratory poem: ODE.
54. Suitable: APT.
55. Detergent brand: PUREX.
58. Tiny pest: GNAT.
59. Magna __: CARTA.
60. __ for the night: PUT UP.
61. Lenten symbol: ASHES.
63. Knee parts, for short: ACLS.
65. "The Lord of the Rings" monsters: ORCS.
66. Young horses: FOALS.
67. Spiritual carving: TOTEM.
68. Sportages and Rios: KIAS.
69. Italian scooter: VESPA.
70. Letter-shaped girders: I RAILS. I-shaped girders.
75. "Us too!": SO DO WE.
76. Online source of red carpet coverage: E NEWS.
79. Udder part: TEAT.
80. "You wish!": ASIF.
86. Travel blogger's succinct review: A MUST SEE.
87. Ranchers' ropes: LARIATS.
88. "Le __" : Chic song: FREAK.
90. Lumberjack contest that requires a lot of balance: ROLEO.
91. Paid the full tab: TREATED.
92. Make airtight using thermoplastics, say: HEAT SEAL.
95. Athlete who plays home games near the Great Salt Lake: UTE.
96. Like much battlefield equipment: ARMORED.
98. Like sales not made online: IN STORE.
100. McIlroy's org.: PGA. Rory.
105. Chain material: METAL.
107. Catches fly balls: SHAGS.
108. See-__: THRU.
109. Honey producers: BEES.
111. Perspiration outlet: PORE.
112. Miss a few letters when speaking: SLUR.
114. Email button: SEND.
115. Greek Mars: ARES.
117. Salted part of a margarita glass: RIM.
120. Pastrami sandwich bread: RYE.
C.C.
