This US city was stylish in the past (7)
I believe the answer is:
chicago
'this us city' is the definition.
(I've seen this before)
'stylish in the past' is the wordplay.
'stylish' becomes 'chic' (similar in meaning).
'in the past' becomes 'ago' (synonyms).
'chic'+'ago'='CHICAGO'
'was' is the link.
(Other definitions for chicago that I've seen before include "A Windy City in Illinois" , "3 city in the US" , "American city; musical" , "Musical (in Illinois?)" , "1970s' John Kander/Fred Ebb musical" .)