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 (most but not all of the word).
'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" .)