City formerly known as Constantinople (8)
I believe the answer is:
istanbul
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">city formerly known as constantinople</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 istanbul that I've seen before include "Turkish port city" , "Modern name of Constantinople" , "City straddling the Bosporus" , "Port and former capital of Turkey" , "Catch the Latin bus out to Turkey" .)