Wood mostly for police (4)
I believe the answer is:
'for police' is the definition.
(cop can informally mean a police officer)
'wood mostly' is the wordplay.
'wood' becomes 'copse' (a copse is a small wood).
'mostly' means to remove the last letter.
'copse' with its last letter taken away is 'COPS'.
(Other definitions for cops that I've seen before include "Police (slang)" , "Familiar American policemen" , "The police (sl.)" , "Panda owners" .)