1. Pop music's Lady ___ → GAGA
5. Kid → CHILD
10. Sandwich shop → DELI
14. Nerve fiber → AXON
15. Product of a grapevine? → RUMOR
16. Company bigwig → EXEC
17. Opposite of surplus → DEBT
18. Kind of acid that is the building block for a protein → AMINO
19. Something you click that starts "http" → LINK
20. Comfortably well-off → SITTINGPRETTY
23. Dashboard reading, for short → RPM
25. Top color in a rainbow → RED
26. Pecan or pistachio → NUT
27. Rub out → ERASE
29. What you might drive while your car is in the shop → LOANER
32. Cereal with "marbits" and a leprechaun mascot → LUCKYCHARMS
35. Above, poetically → OER
38. Like pen smudges → INKY
39. Wall for an outdoor maze → HEDGE
40. Italian volcano → ETNA
41. A white one is a wardrobe staple → TEE
42. Toy that comes out of a spray can → SILLYSTRING
44. Horse with a spotted coat → DAPPLE
46. What The Week puts out once a week → ISSUE
47. Result of putting two and two together → SUM
48. Off-roader, in brief → ATV
50. What Bart Simpson sometimes calls Lisa → SIS
51. Fowl play? ... or words that can follow the starts of 20-, 32- and 42-Across → DUCKDUCKGOOSE
57. Universal building block → ATOM
58. "Nuh-uh!" → NOWAY
59. Enclose in fancy paper, say → WRAP
62. Root vegetable used for poi → TARO
63. Playing card better known as a jack → KNAVE
64. Classroom helper → AIDE
65. [I'm standing right here!] → AHEM
66. Went downhill or cross-country, say → SKIED
67. Cat call → MEOW