Verification of alcohol strength (5)
I believe the answer is:
proof
This clue is a double definition.
'verification' is the first definition.
(thesaurus)
'alcohol strength' is the second definition.
(I've seen this before)
'of' acts as a link.
(Other definitions for proof that I've seen before include "Evidence that establishes the truth" , "Exposition ending QED?" , "Conclusive evidence or verification" , "Decisive evidence" , "to give a watertight case?" .)