Top numbers, each unique (4)

I believe the answer is:

ones

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

'top numbers' is the definition.
'ones' can be an answer for 'numbers' (one is a kind of number). I'm unsure of the 'top' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for ones that I've seen before include "(Of ship) returning to port of origin" , "Singles" , "Aces" , "Units" , "1 & 1 & 1" .)

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