Poisonous substance (5)
I believe the answer is:
toxin
'poisonous substance' is the definition.
(I know that toxin is a type of poisonous substance)
This is the entire clue.
(Other definitions for toxin that I've seen before include "Form of poison" , "Poison, venom" , "Poisonous substance" , "Poison of organic origin" , "Natural poison" .)