Hasten to change capital (6)
I believe the answer is:
athens
'capital' is the definition.
(capital of Greece)
'hasten to change' is the wordplay.
'to change' indicates an anagram.
'hasten' anagrammed gives 'ATHENS'.
(Other definitions for athens that I've seen before include "Capital of Greece" , "EU capital" , "City of the Parthenon" , "Hasten to the Hellenic city" , "City in which 13 Across founded school" .)