Old Greek citizen in the bar (6)
I believe the answer is:
theban
'old greek citizen' is the definition.
(Thebes is an ancient Greek city)
'the bar' is the wordplay.
'bar' becomes 'ban' (both can mean to outlaw or block).
'the'+'ban'='THEBAN'
'in' acts as a link.
(Other definitions for theban that I've seen before include "Native of one ancient Greek or Egyptian city" , "ancient Greek citizen" , "Inhabitant of an ancient Egyptian capital" , "Relating to an ancient Greek city" , "Old citizen" .)