"'39" is a criminally underrated 21A21AGroup composed most famously of 1A, 10A, 39A and 40A [-----] song. Love that you chose it as the title. Great puzzle!
Angolan currency that's an African-American celebration cut short?
21A
Group composed most famously of 1A1AChurch official[------], 10A10ASol's smallest planet[-------], 39A39AMight perhaps[---] and 40A40ASwift or Lautner[------]
23A
How Walter Donovan chose at the end of Indiana Jones and the Last Crusade
26A
Musical duo connector, often
29A
It comes at ___ (has complications)
30A
Live ___ (1985 concert feat. 21A21AGroup composed most famously of 1A, 10A, 39A and 40A [-----])
32A
Day dreaming?
33A
Check again after seasoning
37A
Disease linked to the viral Ice Bucket Challenge (Abbr.)
38A
Bring to life
39A
Might perhaps
40A
Swift or Lautner
Down
1D
Privately connected with, online
2D
Spookily strange
3D
First word of The Beach Boys' "Kokomo"
4D
Light sensor tech used in early SLRs, and in astrophotography
5D
Yves' "Yes!"?
6D
IATA code for Tokyo's Narita International Airport
7D
Possible response to a bad pun
8D
"Malcolm in the Middle" star Frankie
9D
Athenian marketplace
11D
"___ can!" (2008 slogan)
17D
= Opposite Side / Adjacent Side [for the Angle A]
19D
OT interval?
20D
Put it in doors!
22D
Lead-in to sonic or violet
23D
Defunct global airline ... or defunct TV show about said global airline