Crossword Genius

A big one won't pop (3)

Ross

I believe the answer is:

gun

I'm a little stuck... Click here to teach me more about this clue!

'pop' is the definition.
The answer and definition can be both related to competing as well as being verbs in their base form.
Maybe there's a link between them I don't understand?

'a big one won't' is the wordplay.
I cannot quite see how this works, but
'one' could be 'un' (northern English dialect form of 'one') and 'un' is found in the answer.
A single letter 'g' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for gun that I've seen before include "Shooter" , "Artillery piece" , "An offensive weapon" , "It can be carried by 15 Ac" , "Arm" .)

I've seen this clue in the Evening Standard.
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