Jack leaves wound to harden (5)

I believe the answer is:

inure

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'to harden' is the definition.
('inure' can be a synonym of 'harden')

I can't explain the rest of the clue.

(Other definitions for inure that I've seen before include "Come to accept" , "Accustom (someone) to something unpleasant" , "Get used to conditions" , "Cause to become hardened to" , "Steel" .)

I've seen this clue in The Guardian.

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