Californian officers go back for instrument (6)
I believe the answer is:
lapdog
'instrument' is the definition.
(both can mean a person used by another)
'californian officers go back' is the wordplay.
'californian officers' becomes 'lapd' (Los Angeles Police Department).
'back' shows that the letters should be reversed in order.
'go' backwards is 'og'.
'lapd'+'og'='LAPDOG'
'for' is the link.
(Other definitions for lapdog that I've seen before include "One docile and subservient" , "Small pet - sycophant" , "devoted supporter" , "little pet" , "Pom, for one" .)