A natural restorative (5)

I believe the answer is:

sleep

I'm a little stuck... Click here to teach me more about this clue!

'a natural restorative' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the 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.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.