Saint high on life, ultimately blessed, starts back in big city (8)
I believe the answer is:
Here is my best explanation:
'big city' is the definition.
'saint high on life ultimately blessed starts back' is the subsidiary indication.
'high' is an anagram indicator (can mean drunk or on drugs and hence disordered).
'on' is a charade indicator (letters next to each other).
'starts' suggests taking the first letters.
'back' says the letters should be written backwards.
The initial letters of 'life ultimately blessed' is 'lub'.
'lub' written backwards gives 'bul'.
'saint' with letters rearranged gives 'istan'.
'in' is the link.
(Other definitions for istanbul that I've seen before include "foreign city", "byzantium", "Former Turkish capital", "European city", "Nail bust in the Turkish city", "City straddling the Bosporus", "foreign port", "Turkey's largest city", "City - lab units (anag)" and "Eurasian city".)