Biscuit enthusiast (3)

I believe the answer is:

nut

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

This might be a double definition.

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

'enthusiast' is the second definition.
(nut can mean an enthusiast or fan)

Can you help me to learn more?

(Other definitions for nut that I've seen before include "Bolt counterpart" , "1A [CRACKPOT]?" , "something turned when making fast > food" , "Cashew, for example" , "Hard shell of a locking device" .)

I've seen this clue in The Times.

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