Constituent in South Africa in ancient city (6)
I believe the answer is:
sparta
'ancient city' is the definition.
(I've seen this before)
'constituent in south africa' is the wordplay.
'constituent' becomes 'part' (I've seen this before).
'in' means one lot of letters goes inside another.
'south africa' becomes 'sa' (abbreviation).
'part' put within 'sa' is 'SPARTA'.
'in' acts as a link.
(Other definitions for sparta that I've seen before include "Ancient kingdom renowned for asceticism" , "Ancient Greek city where great self-denial was practised" , "Ancient Greek city state" , "Ancient Greek city noted for tough citizens" , "Ancient Greek city, capital of Laconia" .)