Loan shark certain to be found in ancient city (6)
I believe the answer is:
usurer
'loan shark' is the definition.
('usurer' can be a synonym of 'loan shark')
'certain to be found in ancient city' is the wordplay.
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'to be found in' means one lot of letters goes inside another.
'ancient city' becomes 'ur'.
'sure' going inside 'ur' is 'USURER'.
(Other definitions for usurer that I've seen before include "Shylock, say" , "Moneylender who charges excessive rates of interest" , "Unscrupulous lender" , "Grasping money lender" , "Businessman taking a lot of interest" .)