Crossword Genius

Question about energy that goes into cheese-making (4)

Ross

I believe the answer is:

whey

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'question about energy that goes into cheese-making' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'question about energy' is the wordplay.
'question' becomes 'why' (why is a question word).
'about' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'why' going around 'e' is 'WHEY'.

Can you help me to learn more?

(Other definitions for whey that I've seen before include "Separated part of milk" , "Residual liquid from clotted milk" , "Watery residue of milk" , "Watery dairy product" , "11 Across residue" .)

I've seen this clue in The Guardian.
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