Evidence of alcohol (5)
I believe the answer is:
'evidence of alcohol' is the definition.
'proof' can be an answer for 'evidence' (evidence proves something). I am not sure about the 'of alcohol' bit.
'evidence of' is the wordplay.
'evidence' becomes 'pro' (pro is a kind of evidence).
Can you help me to learn more?
(Other definitions for proof that I've seen before include "Specimen", "may need amending", "Author may have to check this", "Testimony", "mathematical tour de force".)