Levy held up by Cornish banker dealing with outmoded system (6)
I believe the answer is:
feudal
'with outmoded system' is the definition.
(as in a feudal social structure)
I cannot understand how the rest of the clue works.
(Other definitions for feudal that I've seen before include "Medieval in approach" , "Resembling or relating to a medieval social system" , "Relating to land held on condition of service" , "Of times of vassals and lords" , "Of system of land-holding in Middle Ages" .)