Moneylender certain to be in the ancient city (6)
I believe the answer is:
usurer
'moneylender' is the definition.
(a usurer is an exploitative moneylender)
'certain to be in the ancient city' is the wordplay.
'certain' becomes 'sure' (synonyms).
'to be in the' means one lot of letters goes inside another.
'ancient city' becomes 'ur' (ancient Sumerian city state).
'sure' put into 'ur' is 'USURER'.
(Other definitions for usurer that I've seen before include "Lender who charges exorbitant interest rates" , "Unrestrained money lender" , "Grasping money-lender" , "Extortionate lender of money" , "Moneylender who charges very high interest" .)