Sound-boosting gear (4)
I believe the answer is:
amps
'sound-boosting gear' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for amps that I've seen before include "Basic units of electric current" , "concert gear" , "'Units of electric current, in short (4)'" , "Spam (anag)" , "Current measures" .)