Box, thanks to Greek city state (6)
I believe the answer is:
sparta
'city state' is the definition.
(I know that Sparta is a city state)
'box thanks' is the wordplay.
'box' becomes 'spar' (spar can mean to box).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'spar'+'ta'='SPARTA'
'to greek' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for sparta that I've seen before include "Laconic state" , "Ancient Greek city state" , "Ancient Greek city, capital of Laconia" , "place of austerity" , "Ancient Greek city-state" .)