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