Biscuit treatise showing where food goes (9,5)

I believe the answer is:

digestive tract

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'where food goes' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'biscuit treatise' is the wordplay.
'biscuit' becomes 'digestive' (type of biscuit).
'treatise' becomes 'tract' (tract is a kind of treatise).
'digestive'+'tract'='DIGESTIVE TRACT'

'showing' is the link.

Can you help me to learn more?

(Another definition for digestive tract that I've seen is " Alimentary canal".)

I've seen this clue in The Daily Express.

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