Citadel of ancient Greece (9)
I believe the answer is:
acropolis
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">citadel of ancient greece</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)