Important old city gent (6)
I believe the answer is:
urgent
'important' is the definition.
(I've seen this before)
'old city gent' is the wordplay.
'old city' becomes 'ur' (ancient city state).
'ur'+'gent'='URGENT'
(Other definitions for urgent that I've seen before include "To be done asap" , "Of immediate importance" , "Pressing, immediate" , "Pressing, important" , "Requiring immediate action" .)