A big one won't pop (3)
I believe the answer is:
gun
'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" .)