Criminal swapping one key for another — one's bound to work (7)

I believe the answer is:

villein

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'criminal' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe you can see a link between them that I can't see?

I can't explain the remainder of the clue.

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(Other definitions for villein that I've seen before include "vassal" , "Subject" , "Lowly worker" , "Feudal peasant" , "Feudal tenant" .)

I've seen this clue in The Times.

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