They send out Mister Messy to collect note (8)

I believe the answer is:

emitters

'they send out' is the definition.

'mister messy to collect note' is the wordplay.
'messy' indicates an anagram.
'to collect' is an insertion indicator.
'note' becomes 'te' (musical note).
'mister' anagrammed gives 'emitrs'.
'emitrs' placed around 'te' is 'EMITTERS'.

I've seen this clue in The Guardian.

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