Bucket refilled below not unique (9)

I believe the answer is:

nonpareil

I'm a little stuck... Click here to teach me more about this clue!

'unique' is the definition.
(I've seen this before)

'bucket refilled below not' is the wordplay.
I cannot quite see how this works, but
'not' could be 'non' (I've seen this in other clues) and 'non' is found within the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for nonpareil that I've seen before include "Unique type" , "unmatched!" , "Unequalled" , "Super model" , "A variety of apple" .)

I've seen this clue in The Daily Express.

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