Superior stimulant (5)
I believe the answer is:
upper
I believe this is a double definition.
'superior' is the first definition.
(thesaurus)
'stimulant' is the second definition.
(thesaurus)
(Other definitions for upper that I've seen before include "Higher; part of boot" , "Nearer the top" , "Higher; part of shoe" , "catcher of swans?" , "form of stimulant" .)