Terribly sure about old city loan shark (6)
I believe the answer is:
usurer
'loan shark' is the definition.
(I know that loan shark can be written as usurer)
'terribly sure about old city' is the wordplay.
'terribly' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'old city' becomes 'ur' (ancient city state).
'sure' anagrammed gives 'user'.
'user' placed around 'ur' is 'USURER'.
(Other definitions for usurer that I've seen before include "Moneylender who charges exorbitant rates" , "One who charges excessive interest" , "Moneylender who charges excessive rates of interest" , "Businessman taking a lot of interest" , "Moneylender who charges very high interest" .)