Unusually half-hearted copper isn't a senior officer (9)
I believe the answer is:
inspector
'officer' is the definition.
(inspector is a kind of officer)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for inspector that I've seen before include "Bus, train official" , "Police officer" , "Ticket examiner" , "Examining official" , "Person whose job it is to examine efficiency as in schools" .)