Bit of band equipment (3)
I believe the answer is:
amp
'bit of band equipment' 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 is increased by this" , "Sound booster" , "Device making sound louder" , "Device to up the sound of an electric guitar (abbr)" , "measure of power" .)