Sound system part (3)
I believe the answer is:
amp
'sound system part' 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 "Sound-boosting piece of equipment (abbr)" , "Unit of electricity, in short" , "Map of electrical unit" , "Device making sound louder" , "Device to up the sound of an electric guitar (abbr)" .)