Cold is able to disappear after knocking back alcohol (6)
I believe the answer is:
cognac
'alcohol' is the definition.
(I know that Cognac is a alcohol)
'cold is able to disappear after knocking back' is the wordplay.
'cold' becomes 'c' (eg on taps).
'is able to' becomes 'can' (if you can do something you are able to do it).
'disappear' becomes 'go' (going is a kind of disappearing).
'after' means one lot of letters go next to another.
'knocking back' is a reversal indicator.
'can'+'go'='cango'
'cango' reversed gives 'ognac'.
'c'+'ognac'='COGNAC'
(Other definitions for cognac that I've seen before include "High quality brandy" , "Brandy of the French Charente" , "(French district famed for) fine brandy" , "Good brandy" , "Type of brandy" .)