Imitate policeman going to Yard (4)
I believe the answer is:
copy
'imitate' is the definition.
(synonyms)
'policeman going to yard' is the wordplay.
'policeman' becomes 'cop' (cop is a kind of policeman).
'going to' is a charade indicator (letters next to each other).
'yard' becomes 'y'.
'cop'+'y'='COPY'
(Other definitions for copy that I've seen before include "Unoriginal" , "Crib" , "Facsimile" , "stuff to be published" , "Duplicate" .)