Theme: S'z - Interesting. Drop the ending ES sound of a common phrase and replace it with an actual S and the ending sound is then closer to a Z. Then clue accordingly.
20A. Shrine to wild animal parks? : TEMPLE OF ZOOS. zoo(z) - Zeus
33A. Cows' reactions to having their hair and makeup done? : STYLING MOOS. moo(z) - mousse
41A. Trashing toilets in London? : BUSTING LOOS. loo(z) - loose
51A. Bake mud pies? : COOK ONE'S GOOS. goo(z) - goose
Argyle here. That is my take on the theme; Marti will explain it, I'm sure. The double O's may figure in there, also.
1. [I'm shocked!] : [GASP!]. The brackets indicate a sound rather than a spoken word.
5. "Who's on First?" catcher : TODAY. "I don't know" - third base.
10. Ad writer's award : CLIO. CLIO was a Greek muse. about Clio.
14. __ of passage : RITE
15. Coeur d'__, Idaho : ALENE. Map It's up in the pan handle or, as some prefer, North Idaho.
16. Miami hoopsters : HEAT
17. Concept, in Cannes : IDÉE. French.
18. Contemporary of Byron and Shelley : KEATS. The three Romantic poets of the early nineteenth century.
19. Points the rifle : AIMS
23. Red __: cinnamon candies : HOTS. "Hot enough for you?"
24. "__ Room": old TV show for preschoolers : ROMPER
If you want to learn more, go to this site.
28. On the beach : ASHORE
31. Econ. measure : GNP. (Gross National Product) Not the same as Gross Domestic Product. JzB, please explain.
32. Mimic : APE
36. Place for a margarita's salt : RIM
37. Reel-to-reel medium : TAPE
38. City area, briefly : URB
39. 451, in old Rome : CDLI. To over the road truckers, this would parse: Commercial Drivers License, Class 1.
40. Haile Selassie's land: Abbr. : ETH. (Ethiopia)
45. Regret : RUE
46. Prov. bordering Quebec : ONT. Ontario Map.
47. Round gaskets : O-RINGS
48. Guard at the gate : SENTRY
50. "Let's call __ evening" : IT AN. Not yet.
57. Ivy, e.g. : VINE
60. Roo's mom : KANGA
61. Cookbook author Rombauer : IRMA
62. Nefarious doings : EVIL
63. St. __ fire : ELMO'S
64. Grandma : NANA
65. Be a snitch : TELL
66. Run-down : SEEDY
67. Smooth-tongued : GLIB
1. "True __": John Wayne film : GRIT. U.S. Marshal Reuben J. "Rooster" Cogburn.
2. Classroom assistant : AIDE
3. Wineglass feature : STEM
4. Way to see through a door : PEEPHOLE
5. Go on a break : TAKE TEN. For when "Take Five" won't do. Take Ten.(3:12)
6. Bread spreads : OLEOs
7. Refusing to listen : DEAF. Clue seems to be asking for a verb.
8. 1998 animated bug movie : "ANTZ". Woody Allen as a neurotic ant. (Of course.)
9. "Make up your mind!" : "YES OR NO?"
10. Deep cleft : CHASM
11. Waikiki welcome : LEI
12. Sam-__: Seuss character : I AM
13. NBA tiebreakers : OT's. (overtime)
21. Petty of "Tank Girl" : LORI. Her IMDb site.
22. Klutz's cry : "OOPS!"
25. Convict's absolution from the governor : PARDON
26. Literary postscript : EPILOG. I remember the TV series, "The Fugitive", always ended with an epilog.
27. Negligent : REMISS
28. Colorful fall flowers : ASTERS
29. Michelangelo's David, for one : STATUE
30. - : HYPHEN
31. Mongolian desert : GOBI
34. Sudden wind : GUST
35. "The A-Team" muscleman : MR. T
39. Desperately hanging on : CLINGING
41. NYC division, briefly : BORO
42. Disconnects, as oxen : UNYOKES
43. Like "bein' green," to Kermit : NOT EASY
44. Mardi __ : GRAS
49. Immune system lymphocyte : T-CELL. Named after Mr. T.
50. Coin phrase beginning : IN GOD
52. Curly cabbage : KALE
53. "My treat!" : "ON ME!"
54. Spoken : ORAL
55. Prefix with potent : OMNI
56. Swedish automaker : SAAB. Former Swedish automaker. Omni was a former Dodge compact.
57. Nov. 11 honoree : VET
58. "__ had enough!" : I'VE. Time to wrap this up. There's nothing after this.
59. Zip : NIL
Added later (C.C.):
Congratulations to Spitzboov & his lovely wife Betty on their 47th wedding anniversary!