One taking a lot of interest in ancient city, certain to be impressed (6)
I believe the answer is:
usurer
'one taking a lot of interest' is the definition.
'ancient city certain to be impressed' is the wordplay.
'ancient city' becomes 'ur'.
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'to be impressed' means one lot of letters goes inside another (impress historically meant to conscript).
'ur' going around 'sure' is 'USURER'.
'in' is the link.
(Other definitions for usurer that I've seen before include "Businessman taking a lot of interest" , "Lender who charges exorbitant interest rates" , "Unrestrained money lender" , "Person who charges excessive interest on loan" , "Moneylender who charges excessive rates of interest" .)