One who'd accept everything was well or fantastic (9)

I believe the answer is:

swallower

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'one who'd accept everything' is the definition.
I can't tell whether this definition defines the answer.

'was well or fantastic' is the wordplay.
'fantastic' indicates anagramming the letters (a fantastic or improbable ordering of the letters).
'was'+'well'+'or'='waswellor'
'waswellor' is an anagram of 'SWALLOWER'.

Can you help me to learn more?

(Another definition for swallower that I've seen is " See 18".)

I've seen this clue in The Independent.

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