Group in nightgear messed around (9)

I believe the answer is:

gathering

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

'group in' is the definition.
'gathering' can be an answer for 'group' (thesaurus). I am not sure about the 'in' bit.

'nightgear messed around' is the wordplay.
'nightgear' can be anagrammed to 'GATHERING'.
Though, I'm unsure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for gathering that I've seen before include "Collecting" , "Assembly; a boil" , "Sewing" , "party" , "convention?" .)

I've seen this clue in The Telegraph.

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