Crossword Genius

Cream derived from goat grease (9)

Ross

I believe the answer is:

butterfat

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

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

'goat grease' is the wordplay.
'goat' becomes 'butter' (something which 'butts').
'grease' becomes 'fat'.
'butter'+'fat'='BUTTERFAT'

'derived from' is the link.

Can you help me to learn more?

(Other definitions for butterfat that I've seen before include "Ingredient of milk" , "Dairy product" .)

I've seen this clue in the Sydney Morning Herald.
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