City formerly known as Constantinople (8)
I believe the answer is:
istanbul
'city formerly known as constantinople' is the definition.
(I've seen this before)
This is all the clue.
(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" .)