Code word for E in city house (4)
I believe the answer is:
echo
'code word for e' is the definition.
(I've seen this in another clue)
'city house' is the wordplay.
'city' becomes 'EC' (the City of London postcode).
'house' becomes 'ho' (abbreviation for house).
'ec'+'ho'='ECHO'
'in' acts as a link.
(Other definitions for echo that I've seen before include "Reminder" , "Reflected radar beam" , "Sound returning some moments later" , "Greek girl in love with Narcissus" , "Return of sound waves" .)