Smart answer -- leave city! (7)
I believe the answer is:
chicago
'city' is the definition.
(Chicago is an example)
'smart answer leave' is the wordplay.
'smart' becomes 'chic' ('chic' can be a synonym of 'smart').
'answer' becomes 'a' (abbreviation eg in Q and A).
'leave' becomes 'go'.
'chic'+'a'+'go'='CHICAGO'
(Other definitions for chicago that I've seen before include "Port on Lake Michigan" , "Windy City of USA" , "1970s' John Kander/Fred Ebb musical" , "A windy place" , "US city - musical featuring All That Jazz" .)