Popular middleman to hang loose, flustered (2,1,4)

I believe the answer is:

in a flap

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

'middleman to hang loose flustered' is the definition.
I know nothing about this answer so I can't judge whether this works.

'popular' is the wordplay.
I cannot really understand how this works, but
'popular' could be 'in' (something that's 'in' is fashionable) and 'in' is present in the answer.
This explanation may well be incorrect...

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I've seen this clue in The Irish Times.

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