Crossword Genius

Jack, who doffs his cap in church space (5)

Ross

I believe the answer is:

knave

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'jack' is the definition.
(knave can mean a jack playing card)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for knave that I've seen before include "Jack - the cad!" , "Honour" , "A swindler" , "cheat of old" , "Rat" .)

I've seen this clue in The Guardian.
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