City house is sound again (4)
I believe the answer is:
echo
'is sound again' is the definition.
(I've seen this before)
'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 "The distant sound" , "Imitate -- a nymph" , "Sound following" , "Reflection of sound wave" , "Mythical nymph" .)