You could hit a punch-bag with one (4)
I believe the answer is:
thud
'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.
Can you help me to learn more?
(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" .)