Citadel of ancient Greece (9)
I believe the answer is:
acropolis
'citadel of ancient greece' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for acropolis that I've seen before include "Upper fortified part of Greek city - Athens's is famous" , "Citadel of ancient Greece" , "place of high security" , "Athens landmark" , "old citadel" .)