Crossword Genius

Riotous party breaking up (4,9)

Ross

I believe the answer is:

side splitting

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But I can't explain it at all! Could you help me understand?

(Other definitions for side splitting that I've seen before include "Hysterical" , "a scream!" , "Extremely funny" , "Really funny" , "Hilarious" .)

I've seen this clue in The Hindu.
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