Instance of longevity, diagnosing no upsets (1,4,7)

I believe the answer is:

a good innings

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'instance of longevity' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'diagnosing no upsets' is the wordplay.
'upsets' indicates anagramming the letters.
'diagnosing'+'no'='diagnosingno'
'diagnosingno' with letters rearranged gives 'A GOOD INNINGS'.

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I've seen this clue in The Everyman puzzle in The Observer.

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