O constant plecity? (8)
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" .)