US city abandoning past - that's smart (4)
I believe the answer is:
chic
'that's smart' is the definition.
(both can mean stylish)
'us city abandoning past' is the wordplay.
'us city' becomes 'chicago' (Chicago is a US city).
'abandoning' indicates named letters should be taken away.
'past' becomes 'ago' (similar in meaning).
'chicago' with 'ago' taken away is 'CHIC'.
(Other definitions for chic that I've seen before include "Smart - elegant - fashionable" , "Trendy" , "Elegantly fashionable" , "Stylish and elegant - in France" , "Stylishness, elegance" .)