Its strength can be shown by proof (7)

I believe the answer is:

alcohol

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'proof' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for alcohol that I've seen before include "Intoxicating liquour" , "The hard stuff" , "drinks" , "To a position aboard" , "Intoxicating drink" .)

I've seen this clue in The Globe and Mail.

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