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 cannot understand how they can define each other.

This is all the clue.

Can you help me to learn more?

(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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