Poisonous mushroom (9)
I believe the answer is:
toadstool
'poisonous mushroom' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for toadstool that I've seen before include "Umbrella-shaped fungus" , "Fungal growth" , "Inedible fungus" , "Common fungus" , "a potentially toxic development" .)