In the book for a punch-up? (5)

I believe the answer is:

scrap

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'in' is the definition.
Both the answer and definition are adjectives. Maybe there's a link between them I don't understand?

I don't understand how the remainder of the clue works.

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(Other definitions for scrap that I've seen before include "Turn into refuse" , "Break up - fight" , "Just a fragment, maybe of food" , "Tussle over junk metal?" , "Junk; altercation" .)

I've seen this clue in the Evening Standard.

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