Most tasteless supporter backed celebratory gathering (7)

I believe the answer is:

naffest

'most tasteless' is the definition.
(naff can mean tasteless or of low quality)

'supporter backed celebratory gathering' is the wordplay.
'supporter' becomes 'fan' (fan is a kind of supporter).
'backed' says the letters should be written in reverse.
'celebratory gathering' becomes 'fest' (festival).
'fan' backwards is 'naf'.
'naf'+'fest'='NAFFEST'

(Another definition for naffest that I've seen is " in the worst possible taste".)

I've seen this clue in The Times.

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