Possibly form twenty per cent (5)

I believe the answer is:

fifth

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

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(Other definitions for fifth that I've seen before include "taken by Americans in self-defence" , "20 per cent" , "Fraction" , "US amendment guaranteeing the right to silence" , "- Column is a group working for the enemy" .)

I've seen this clue in the Evening Standard.

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