Rock-band boxes (4)
I believe the answer is:
amps
'rock-band boxes' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for amps that I've seen before include "Units of electrical power" , "Current measures" , "'Units of electric current, in short (4)'" , "concert gear" , "Spam (anag)" .)