Full ones are best (5)

I believe the answer is:

marks

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'full ones' is the definition.
The answer and definition can be both quantities as well as being plural nouns.
Maybe there's a link between them I don't understand?

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for marks that I've seen before include "Symbols" , "Visible signs" , "Goals" , "Grades assigned to school work" , "points" .)

I've seen this clue in the Evening Standard.

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