Sound boosters from Hampstead (4)
I believe the answer is:
amps
'sound boosters' is the definition.
(I've seen this in another clue)
'from hampstead' is the wordplay.
'from' indicates a hidden word.
'AMPS' is hidden in the letters of 'hampstead'.
(Other definitions for amps that I've seen before include "concert gear" , "Units of electrical power" , "Current measures" , "Basic units of electric current" , "'Units of electric current, in short (4)'" .)