Sound-boosting device (3)
I believe the answer is:
amp
'sound-boosting device' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for amp that I've seen before include "In short, unit for measuring electric current" , "Sound-boosting piece of equipment (abbr)" , "Map about electric unit" , "sound is increased by this" , "Map of electric current" .)