Smartness leading to a success in US city (7)
I believe the answer is:
chicago
'us city' is the definition.
(I've seen this before)
'smartness leading to a success' is the wordplay.
'smartness' becomes 'chic' ('chic' can be a synonym of 'smartness').
'leading to' says to put letters next to each other (some letters go before or lead others).
'success' becomes 'go' (I've seen this in other clues).
'chic'+'a'+'go'='CHICAGO'
'in' is the link.
(Other definitions for chicago that I've seen before include "American city; musical" , "3 city in the US" , "Illinois port" , "A Windy City in Illinois" , "US city - musical featuring All That Jazz" .)