Acoustic effect needed by the choirmaster (4)
I believe the answer is:
echo
'acoustic effect' is the definition.
(effect made by reflected sound waves)
'needed by the choirmaster' is the wordplay.
'needed by' indicates a hidden word (the solution letters are needed to write the clue).
'ECHO' can be found hidden inside 'the choirmaster'.
(Other definitions for echo that I've seen before include "Imitate -- a nymph" , "Reproduction" , "Greek nymph, spurned by Narcissus, who pined away until only her voice remained" , "Nymph (only able to repeat the last words of others)" , "Return of sound waves" .)