This system worked in medieval times (6)

I believe the answer is:

feudal

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'this system' is the definition.
(feudal system is a kind of system)

I don't understand the remainder of the clue.

(Other definitions for feudal that I've seen before include "Of system of land-holding in Middle Ages" , "Pertaining to fiefs" , "Like a medieval social system" , "Pertaining to old social system" , "Medieval in approach" .)

I've seen this clue in The Globe and Mail.

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