LSD, say (12)
I believe the answer is:
hallucinogen
'lsd say' is the definition.
(lsd is a kind of hallucinogen)
This is the entire clue.
(Other definitions for hallucinogen that I've seen before include "Mind-altering drug" , "mind-blowing" , "Mind-affecting drug" , "Drug inducing unreal perceptions" , "Drug such as LSD" .)