Concert booster (3)
I believe the answer is:
amp
'concert booster' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for amp that I've seen before include "Sound booster" , "Sound-boosting piece of equipment (abbr)" , "Device to up the sound of an electric guitar (abbr)" , "Current measure" , "Map about electric unit" .)