Upset when FAI plan goes astray (2,1,4)

I believe the answer is:

in a flap

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'upset' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'fai plan goes astray' is the wordplay.
'goes astray' indicates anagramming the letters.
'fai'+'plan'='faiplan'
'faiplan' anagrammed gives 'IN A FLAP'.

'when' is the link.

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I've seen this clue in The Irish Times.

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