Shylock is certain to be found in the old city (6)
I believe the answer is:
usurer
'shylock' is the definition.
(I know that shylock can be written as usurer)
'certain to be found in the old city' is the wordplay.
'certain' becomes 'sure' (synonyms).
'to be found in' is an insertion indicator.
'the old city' becomes 'ur' (ancient city state).
'sure' put inside 'ur' is 'USURER'.
'is' acts as a link.
(Other definitions for usurer that I've seen before include "Person taking a lot of interest" , "Unfair moneylender" , "Former name for money-lender" , "Unrestrained money lender" , "Grasping money-lender" .)