Retire from the contest - not up to it? (7)

I believe the answer is:

scratch

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'retire' is the definition.
Although both the answer and definition are verbs in their base form, I don't understand how they can define each other.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for scratch that I've seen before include "Slight wound - withdraw from contest" , "Decide not to play" , "having superficial injury" , "minor injury" , "Scrape" .)

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

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