Former name of Istanbul (14)
I believe the answer is:
constantinople
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">former name of istanbul</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 constantinople that I've seen before include "seat for emperor" , "Istanbul formerly" , "capital no longer used" , "Former name of Istanbul" , "place that changed its name" .)