In a kettle, it can be warm, naturally (3)

I believe the answer is:

fur

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'in' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps you can see a link between them that I don't see?

I can't explain the remainder of the clue.

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(Other definitions for fur that I've seen before include "Coating formed on tongue of sick person" , "Animal-skin coat" , "Deposit inside kettles" , "Deposit lining kettles" , "Eg, chinchilla" .)

I've seen this clue in the Evening Standard.

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