Sound booster (3)
I believe the answer is:
amp
'sound booster' 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 "measure of power" , "Device to up the sound of an electric guitar (abbr)" , "Map about electric unit" , "Sound-boosting piece of equipment (abbr)" , "In short, unit for measuring electric current" .)