Reveller said bet on outsider (12)

I believe the answer is:

bacchanalian

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'reveller' is the definition.
(I have seen 'drunken reveller' mean 'bacchanalian' so perhaps 'reveller' could also mean 'bacchanalian')

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for bacchanalian that I've seen before include "Relating to drunken revelry" , "Drunken, orgiastic (revel)" , "drunken reveller" , "description of drunken revels" .)

I've seen this clue in The Telegraph.

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