Bug otherwise trapping police's head officer (9)
I believe the answer is:
inspector
'officer' is the definition.
(I know that inspector is a type of officer)
'bug otherwise trapping police's head' is the wordplay.
'bug' becomes 'insect' (bug can mean an insect).
'otherwise' becomes 'or'.
'trapping' means one lot of letters goes inside another.
'head' suggests taking the first letters.
The first letter of 'polices' is 'p'.
'insect'+'or'='insector'
'insector' enclosing 'p' is 'INSPECTOR'.
(Other definitions for inspector that I've seen before include "Policeman; compliance official" , "Ticket examiner" , "one to check" , "bluebottle!" , "reviewer" .)