Only some of the choirboys return (4)
I believe the answer is:
echo
'return' is the definition.
(I have seen 'Return of sound' mean 'echo' so perhaps 'return' could also mean 'echo')
'only some of the choirboys' is the wordplay.
'only some of' says the answer is hidden in the clue (I've seen 'only some' mean this).
'ECHO' can be found hidden inside 'the choirboys'.
(Other definitions for echo that I've seen before include "Repeated sound" , "repercussion" , "Reflection of sound wave" , "Sound returning some moments later" , "Return of sound waves" .)