Crossword Genius

You could hit a punch-bag with one (4)

Ross

I believe the answer is:

thud

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'you' is the definition.
Both the answer and definition are singular nouns.
Perhaps you can see an association between them that I can't see?

I don't understand the remainder of the clue.

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(Other definitions for thud that I've seen before include "Sound made by impact of heavy objects" , "Dull sound made by a falling body" , "Sound of heavy blow" , "Sound of a heavy fall" , "Dull sound of something hitting the ground" .)

I've seen this clue in the Evening Standard.
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