US city fashionable in the past (7)
I believe the answer is:
chicago
'us city' is the definition.
(I've seen this before)
'fashionable in the past' is the wordplay.
'fashionable' becomes 'chic' (chic means stylish or fashionable).
'in the past' becomes 'ago' (synonyms).
'chic'+'ago'='CHICAGO'
(Other definitions for chicago that I've seen before include "A windy place" , "A Windy City in Illinois" , "Type of bridge" , "1970s' John Kander/Fred Ebb musical" , "Musical (in Illinois?)" .)