Vengeful trio that is wearing winter outfits (6)

I believe the answer is:

furies

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'winter outfits' is the definition.
Although both the answer and definition are plural nouns, I can't see how one could define the other.

'vengeful trio that is wearing' is the wordplay.
'vengeful trio' becomes 'furs' (I can't justify this - if you can you should believe this answer much more).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'wearing' indicates putting letters inside.
'furs' placed around 'ie' is 'FURIES'.

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I've seen this clue in The Guardian.

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