Perfect rating, of a sort (3)

I believe the answer is:

ten

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'perfect rating' is the definition.
(I've seen this in another clue)

I don't understand how the remainder of the clue works.

(Other definitions for ten that I've seen before include "time" , "How many" , "Base of decimal system" , "honour" , "Cardinal number" .)

I've seen this clue in the Evening Standard.

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