Too unfunny hiding something up one's sleeve — I don't want it! (2,5,3)

I believe the answer is:

no thank you

'i don't want it' is the definition.

'too unfunny hiding something up one's sleeve' is the wordplay.
I cannot quite see how this works, but
'one' could be 'an' (I've seen this in other clues) and 'an' is present in the answer.
This explanation may well be incorrect...

I've seen this clue in The Guardian.

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