One taking a lot of interest to be certain in old city (6)
I believe the answer is:
usurer
'one taking a lot of interest' is the definition.
'certain in old city' is the wordplay.
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'in' is an insertion indicator.
'old city' becomes 'ur' (ancient city state).
'sure' placed within 'ur' is 'USURER'.
'to be' acts as a link.
(Other definitions for usurer that I've seen before include "Unfair moneylender" , "Extortionate moneylender" , "Person taking a lot of interest" , "Unrestrained money lender" , "Shylock, say" .)