A natural restorative (5)

I believe the answer is:

sleep

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'a natural restorative' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

This is the entire clue.

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(Other definitions for sleep that I've seen before include "Produced (egg)" , "Natural state of unconscious rest" , "Snooze" , "Slumber" , "crash" .)

I've seen this clue in the Evening Standard.

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