Slavery forced marginals into perverse customs (7)

I believe the answer is:

serfdom

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'slavery' is the definition.
(I know that Serfdom is a type of slavery)

I cannot understand how the remainder of the clue works.

(Other definitions for serfdom that I've seen before include "State of slavery" , "Feudal labour" , "Feudal worker's state" .)

I've seen this clue in the Sydney Morning Herald.

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