Stimulant drug (5)
I believe the answer is:
upper
'stimulant drug' is the definition.
(I know that upper is a type of stimulant drug)
This is the entire clue.
(Other definitions for upper that I've seen before include "Higher in position; drug" , "Higher; part of boot" , "Part of a shoe; drug" , "catcher of swans?" , "that's found in shoe" .)