Crossword Genius

O constant plecity? (8)

Ross

I believe the answer is:

istanbul

'o' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

I can't explain the remainder of the clue.

(Other definitions for istanbul that I've seen before include "Port and former capital of Turkey" , "Nail bust in the Turkish city" , "Eurasian city" , "City formerly known as Constantinople" , "Former Turkish capital" .)

I've seen this clue in The Guardian.
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