Crossword Genius

At which point our nephew cracked up (9)

Ross

I believe the answer is:

whereupon

'at which point' is the definition.
(I've seen this before)

'our nephew cracked up' is the wordplay.
'cracked up' indicates anagramming the letters (I've seen this in other clues).
'our'+'nephew'='ournephew'
'ournephew' with letters rearranged gives 'WHEREUPON'.

(Another definition for whereupon that I've seen is " At which point".)

I've seen this clue in The Telegraph.
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