Saint high on life, ultimately blessed, starts back in big city (8)
I believe the answer is:
istanbul
'big city' is the definition.
'saint high on life ultimately blessed starts back' is the wordplay.
'high' indicates an anagram (can mean drunk or on drugs and hence disordered).
'on' says to put letters next to each other.
'starts' suggests taking the first letters.
'back' says the letters should be written backwards.
The first letters of 'life ultimately blessed' is 'lub'.
'lub' reversed gives 'bul'.
'saint' anagrammed gives 'istan'.
'istan'+'bul'='ISTANBUL'
'in' is the link.
(Other definitions for istanbul that I've seen before include "City straddling the Bosporus" , "European city" , "City of Turkey" , "It used to be named Constantinople" , "Nail bust in the Turkish city" .)