Like this puzzle you'd mess up on originally (12)

I believe the answer is:

pseudonymous

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'like this puzzle' is the definition.
The answer and definition are not the same part of speech.

'you'd mess up on originally' is the wordplay.
'originally' indicates anagramming the letters.
'youd'+'mess'+'up'+'on'='youdmessupon'
'youdmessupon' anagrammed gives 'PSEUDONYMOUS'.

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