Its strength can be shown by proof (7)
I believe the answer is:
'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" .)