Rival of Athens (6)
I believe the answer is:
sparta
'rival of athens' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sparta that I've seen before include "Old Greek city state, famous for its citizens' austere lifestyle" , "Laconic state" , "it was a hard place" , "Ancient Greek city, capital of Laconia" , "Ancient Greek city where great self-denial was practised" .)