Ancient citadel of Athens (9)
I believe the answer is:
acropolis
'ancient citadel of athens' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for acropolis that I've seen before include "Athens landmark" , "Large old building" , "Ancient citadel of Athens" , "old citadel" , "Parthenon location" .)