Hallucinogenic fungus (5,8)
I believe the answer is:
magic mushroom
'hallucinogenic fungus' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for magic mushroom that I've seen before include "Woodland hallucinogen" , "Fungus containing a hallucinogenic substance" , "Fungus causing hallucinations" , "Hallucinatory item" , "Fungus with hallucinogenic properties" .)