Stimulant blows head off drinker! (5)
I believe the answer is:
Here is my best explanation:
'stimulant' is the definition.
'blows head off drinker' is the wordplay.
'blows head off' means to remove the first letter.
'drinker' becomes 'supper' (someone who 'sups').
'supper' with its initial letter removed is 'UPPER'.
(Other definitions for upper that I've seen before include "Pep pill", "Higher-ranking", "Illegal stimulant possibly", "Part of a shoe; drug", "Amphetamine pill", "form of stimulant", "Higher in place or position", "Nearer the top", "Higher; more northerly" and "that's found in shoe".)