Moscow citadel (7)
I believe the answer is:
kremlin
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">moscow citadel</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for kremlin that I've seen before include "fortress" , "Castle" , "Moscow landmark" , "Citadel of a Russian town" , "Historic fortified complex in the heart of Moscow" .)