A boxing belt could be the end of a joke (9)

I believe the answer is:

punchline

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'a boxing' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'belt could be the end of a joke' is the wordplay.
'belt' becomes 'punch' (as verb - to belt someone).
'could be' becomes 'lin' (I can't explain this - if you can you should give a lot more credence to this answer).
'the end of' indicates one should take the final letters.
The last letter of 'joke' is 'e'.
'punch'+'lin'+'e'='PUNCHLINE'

Can you help me to learn more?

(Other definitions for punchline that I've seen before include "Part of joke" , "Gag's ending" , "joke leading to row after church" , "end of story" , "Point" .)

I've seen this clue in The Irish Independent.

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