Evidence of alcohol (5)
'evidence of alcohol' is the definition.
'proof' can be an answer for 'evidence' (I've seen this before). 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 "Testimony", "may need amending", "mathematical work", "Author may have to check this", "Draft copy of printed text".)