Explaining why originals ain't fantastic (13)

I believe the answer is:

rationalising

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'explaining why' is the definition.
I can't tell whether this defines the answer.

'originals ain't fantastic' is the wordplay.
'fantastic' is an anagram indicator (a fantastic or improbable ordering of the letters).
'originals'+'aint'='originalsaint'
'originalsaint' anagrammed gives 'RATIONALISING'.

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I've seen this clue in The Irish Times.

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