Sound detectors used in rehearsals (4)
I believe the answer is:
ears
'sound detectors' is the definition.
(I've seen this in another clue)
'in rehearsals' is the wordplay.
'in' indicates a hidden word.
'EARS' is hidden in the letters of 'rehearsals'.
'used' acts as a link.
(Other definitions for ears that I've seen before include "'''Friends, Romans, countrymen, lend me your ...'' (Julius Caesar) (4)'" , "Lugholes" , "Heads of corn" , "Sense organs" , "Sear (anag.)" .)