1. Lose one's footing : SLIP
5. Bruins' sch. : UCLA
9. Sewing machine inventor Howe : ELIAS
14. Dialogue unit : LINE
15. Bridge : SPAN
16. Lone Ranger's sidekick : TONTO
17. 1989 John Hughes movie starring John Candy : UNCLEBUCK
19. Concluding notations : CODAS
20. Took to the slopes : SKIED
21. Accepts punishment unflinchingly : TAKESIT
23. Contains : HAS
24. "There, there ... stop crying" : NOMORETEARS
28. Vain person's problem : EGO
29. Letters before an alias : AKA
30. 35, as a minimum to be U.S. president : AGE
31. "___ on your life!" : NOT
32. Belgrade resident : SERB
34. Volcanic debris : ASH
36. Many a dreadlocks wearer, informally : RASTA
38. Paul McCartney/Michael Jackson hit ... or a hint to the starts of 17-, 24-. 57- and 65-Across : SAYSAYSAY
42. Like one's voice when one has a cold, maybe : NASAL
45. Letter after ess : TEE
46. Biblical garden : EDEN
50. "Incidentally," in a text : BTW
51. The "L" in 5-Across : LOS
54. Spinks foe : ALI
56. Gobbled up : ATE
57. Snack that leaves the fingers orange : CHEESECURLS
60. Puppy's cry : YIP
61. 1996 Mario Puzo novel, with "The" : LASTDON
62. Door fastener : LATCH
64. Vice ___ : VERSA
65. "This is just ridiculous!" : WHATAJOKE
68. Animal cavorting by a stream : OTTER
69. "Mona ___" : LISA
70. Once again : ANEW
71. Boxer Spinks and others : LEONS
72. Tater : SPUD
73. "Darn it all!" : RATS
Down answers are in the link below!