Reactions, perhaps, to a poisonous substance (7)
I believe the answer is:
arsenic
'substance' is the definition.
(arsenic is a kind of substance)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for arsenic that I've seen before include "Can rise result from poison?" , "As briefly identified" , "Ratsbane" , "Element with many poisonous compounds" , "Toxic metallic element used as rat poison" .)