"The Outlaw" - a Greek or Egyptian citizen (6)
I believe the answer is:
'a greek or egyptian citizen' is the definition.
(thebes is in greece and egypt)
'the outlaw' is the wordplay.
'outlaw' becomes 'ban' (banning is a kind of outlawing).
(Other definitions for theban that I've seen before include "Old citizen" , "Nab the person from the ancient Greek city" , "Relating to an ancient Greek city" , "ancient Greek citizen" , "Native of one ancient Greek or Egyptian city" .)