Crossword Genius

One or two dairy products (5,6)

Ross

I think it could be either:

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10

5,6

buttermilk

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

'two dairy products' is the wordplay.
'two dairy products' can mean two substitutions for 'dairy product'.
'dairy product' becomes 'butter' (butter is a kind of dairy product).
'dairy product' becomes 'milk' (milk is a kind of dairy product).
'butter'+'milk'='BUTTERMILK'

'or' acts as a link.

Can you help me to learn more?

(Other definitions for buttermilk that I've seen before include "Residue after churning, used for soda bread" , "Dairy product" , "Liquid used in making soda bread -- I'm Burt Kelt (anag)" .)

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