Copper left in compartment (4)
I believe the answer is:
plod
'copper' is the definition.
(both are slang terms for a police officer)
'left in compartment' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' indicates putting letters inside.
'compartment' becomes 'pod' (pod can mean a compartment or case).
'l' put into 'pod' is 'PLOD'.
(Other definitions for plod that I've seen before include "Trudge - policeman" , "Policeman - walk slowly" , "Tramp along" , "Stomp" , "Walk with heavy steps" .)