A Greek city has ten ruins (6)
I believe the answer is:
athens
'a greek city' is the definition.
(I've seen this before)
'has ten ruins' is the wordplay.
'ruins' is an anagram indicator.
'has'+'ten'='hasten'
'hasten' anagrammed gives 'ATHENS'.
(Other definitions for athens that I've seen before include "European capital city" , "Hasten to Greek city" , "Capital of Greece" , "Hasten to European capital" , "Hen sat down in the city" .)