An equable percentage (5)

I believe the answer is:

fifty

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'an equable percentage' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

This is all the clue.

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(Other definitions for fifty that I've seen before include "Half-century" , "A multiple of ten" , "L" .)

I've seen this clue in the Evening Standard.

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