One’s smart to have earlier left Irish city (7)
I believe the answer is:
lisburn
'irish city' is the definition.
(I've seen this before)
'one's smart to have earlier left' is the wordplay.
'one's' becomes 'is' (Roman numeral).
'smart' becomes 'burn' (burning is a kind of smarting).
'to have earlier' is a charade indicator (letters next to each other).
'left' becomes 'l' (common abbreviation).
'is'+'burn'='isburn'
'isburn' after 'l' is 'LISBURN'.
(Other definitions for lisburn that I've seen before include "Ulster town" , "Irish district" , "Town near Belfast" , "Irish city" .)