Theme: PP - Just two words starting with P? It seems so. Update: "In addition to two P's, each theme entry ends with some kind of animal." Thank you, HeartRx.
17A. Porky's girlfriend : PETUNIA PIG
24A. Feigns sleep, say : PLAYS POSSUM
52A. Either of two of the Inspector Clouseau films, with "The" : PINK PANTHER
61A. Yipping adoptee : POUND PUPPY
Argyle here. Debut constructor...unless it's an anagram.
Across:
1. "Famous Potatoes" state : IDAHO
6. Speak drunkenly : SLUR
10. Addition word : PLUS
14. "__ what?": "What next?" : SO NOW
15. Adhesive strip : TAPE
16. Shopper's memory aid : LIST
19. Impressionist : APER
20. Very __ yours : TRULY
21. Utter mess : SNAFU
22. Tire inflater : AIR
28. Pitt of "Troy" : BRAD
30. Three-note chord : TRIAD
31. Aboveground trains : ELS
32. Per __: for each person, as income : CAPITA
35. Got one's uniform dirty, perhaps : SLID. Baseball. The other timely entry: 4D. Semiannual time-change amount : HOUR
36. Runs away from military duty : DESERTS
38. Israeli parliament : KNESSET. (the gathering)
43. "Exodus" author Leon : URIS. Neat pairing.
45. Haughtily terse : SNIPPY
46. "From __ Zinc": vitamin slogan : A TO
49. Skimpy skirts : MINIs. Do we need to see more? (yes)
51. Cut out, as coupons : CLIP
56. Cooler cubes : ICE OH, NO!
57. World book : ATLAS
58. Like a lummox : INEPT
60. Lamb serving : CHOP
66. Pile : HEAP
67. Undersized 61-Across : RUNT. (pound puppy)
68. Sharp-crested ridge : ARÊTE
69. Novelist Ferber : EDNA
70. Twistable cookie : OREO
71. Leavening agent : YEAST
Down:
1. AOL, for one : ISP. (internet service provider)
2. Deer girl : DOE
3. Devices to stop tiny invading armies : ANT TRAPS
5. Admit (to) : OWN UP
6. Patronize, as a hotel : STAY AT
7. Spot for a cat, or drink like a cat : LAP
8. Wire service initials : UPI. (United Press International)
9. Coffee order: Abbr. : REGular
10. Thinks ahead : PLANS
11. Enzyme that breaks down fats : LIPASE. Is there a pill for that?
12. Handy : USEFUL
13. Plays the banjo, like someone "in the kitchen with Dinah" : STRUMS. Here, I'll sing it for you.
LINK (Montana, this is embedded so I think you shouldn't need the LINK.
18. Unwell : ILL
21. Wetter than wet : SODDEN
22. "The Alphabet Song" start : ABCD
23. "Dies __": Latin hymn : IRAE
25. Mos. and mos. : YRs
26. Fancy tie fabric : SILK
27. "Growing" difficulties : PAINS
29. Craps cube : DIE
33. Spades in a four-spades bridge contract, say : TRUMPS
34. Sunlit courtyards : ATRIA
37. Ireland's __ Féin : SINN. (we ourselves)
39. [error left as is] : [SIC]
40. Soup legume : SPLIT PEA
41. Many a DeMille movie : EPIC
42. Use a keyboard : TYPE
44. Command to Rover : SIT
46. Tribe for which a helicopter is named : APACHE
47. Gave 10% to the church : TITHED
48. Borrowed, as a library book : ON LOAN
50. Japanese religion : SHINTO
53. Phi Beta __ : KAPPA
54. Put a stop to : END
55. Settle, as a debt : REPAY
59. Chaste : PURE
61. NHL player, e.g. : PRO
62. "__ Father, who art ..." : OUR
63. One in Québec : UNE
64. Qt. halves : PTs
65. Nonetheless : YET
Argyle