Stimulant drug (5)
I believe the answer is:
upper
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">stimulant drug</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that upper is a type of stimulant drug)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)