Austere city state once, not all in South Africa (6)
I believe the answer is:
sparta
'austere city state once' is the definition.
(ancient city state famous for its austerity)
'not all in south africa' is the wordplay.
'not all' becomes 'part' ('part' is the opposite to 'all').
'in' is an insertion indicator.
'south africa' becomes 'sa' (abbreviation).
'part' put into 'sa' is 'SPARTA'.
(Other definitions for sparta that I've seen before include "Old Greek city state, famous for its citizens' austere lifestyle" , "Ancient Greek city known for warlike spirit" , "Ancient kingdom renowned for asceticism" , "Formerly "City" , "Ancient Greek city whose citizens were famed for toughness" .)