Convincing evidence of the strength of alcohol (5)
I believe the answer is:
proof
This could be a double definition.
'convincing evidence' is the first definition.
(I've seen this before)
'strength of alcohol' is the second definition.
'proof' can be an answer for 'strength' (I've seen this before). I'm unsure of the 'of alcohol' bit.
'of the' is the link.
Can you help me to learn more?
(Other definitions for proof that I've seen before include "whisky's strength?" , "mathematical tour de force" , "Mathematical argument" , "Specimen" , "Factual evidence" .)