Handle a punch up (4)

I believe the answer is:

knob

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'handle' is the definition.
(I know that knob is a type of handle)

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

(Other definitions for knob that I've seen before include "hard swelling" , "Turn it to open door" , "Ball" , "Round door handle or lump of butter" , "Turning handle" .)

I've seen this clue in the Sydney Morning Herald.

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