Imitate policeman before the Fourth of July (4)
I believe the answer is:
copy
'imitate' is the definition.
(synonyms)
'policeman before the fourth of july' is the wordplay.
'policeman' becomes 'cop' (slang term).
'before' says to put letters next to each other.
'the fourth of july' becomes 'y' (4th letter of 'july').
'cop'+'y'='COPY'
(Other definitions for copy that I've seen before include "Ape; replica" , "Imitation or reproduction" , "Replicate" , "Matter to be printed; ape" , "Closely imitate" .)