Crossword Genius

Sport in which participants struggle, grappling with tiredness? (3-2,9)

Ross

I believe the answer is:

all-in wrestling

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

'sport' is the definition.
(I've seen this before)

'in which participants struggle grappling with tiredness?' is the wordplay.
I cannot quite understand how this works, but
'in' is within the answer.
'struggle' could be 'wrestling' (wrestling is a kind of struggle) and 'wrestling' is found within the answer.
The remaining letters 'all' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Telegraph.
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