US city, smart in the past (7)
I believe the answer is:
chicago
'us city' is the definition.
(I've seen this before)
'smart in the past' is the wordplay.
'smart' becomes 'chic' ('chic' can be a synonym of 'smart').
'in the past' becomes 'ago' ('ago' can be a synonym of 'in the past').
'chic'+'ago'='CHICAGO'
(Other definitions for chicago that I've seen before include "A Windy City in Illinois" , "A windy place" , "American city; musical" , "1970s' John Kander/Fred Ebb musical" , "Musical (in Illinois?)" .)