Crossword Genius

Foremost calories in butter? Every scientist knows that! (4)

Ross

I believe the answer is:

fact

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

This could be a double definition.

'foremost calories in' is the first definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

'butter? every scientist knows that' is the second definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

Can you help me to learn more?

(Other definitions for fact that I've seen before include "Verifiable truth" , "copyright protection agency?" , "An indisputable truth" , "Statement of verifiable information" , "Undisputed truth" .)

I've seen this clue in The Irish Times and the Irish Times (Crosaire).
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