Officers work in Civil Service (4)
I believe the answer is:
cops
'officers' is the definition.
(cop is a kind of officer)
'work in civil service' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'in' is an insertion indicator.
'civil service' becomes 'cs'.
'op' put within 'cs' is 'COPS'.
(Other definitions for cops that I've seen before include "Panda owners" , "Familiar American policemen" , "Police (slang)" , "The police (sl.)" .)