Imitate city house (4)
I believe the answer is:
echo
'imitate' is the definition.
(thesaurus)
'city house' is the wordplay.
'city' becomes 'ec' (the City of London postcode).
'house' becomes 'ho' (abbreviation for house).
'ec'+'ho'='ECHO'
(Other definitions for echo that I've seen before include "Mythical nymph" , "Imitate -- a nymph" , "Be suggestive of" , "Rebounding sound" , "one catches on the rebound?" .)