Drug treated stye sac (7)
I believe the answer is:
ecstasy
'drug' is the definition.
(ecstasy is a kind of drug)
'treated stye sac' is the wordplay.
'treated' indicates an anagram.
'stye'+'sac'='styesac'
'styesac' with letters rearranged gives 'ECSTASY'.
(Other definitions for ecstasy that I've seen before include "Youth drug or unalloyed bliss" , "Bliss, rapture" , "State of euphoria and drug that induces it" , "Great delight" , "State of bliss or euphoria" .)