Concede Istanbul provides sanctuary for believer (5)
I believe the answer is:
deist
'believer' is the definition.
(person who believes in a god)
'concede istanbul provides sanctuary for' is the wordplay.
'provides sanctuary for' says the answer is hidden in the clue.
'DEIST' is hidden within 'concede istanbul'.
(Other definitions for deist that I've seen before include "Religious believer" , "Believer in a creator" , "One who believes" , "Sort of believer" , "Believer in the existence of god" .)