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Reactions, perhaps, to a poisonous substance (7)

Ross

I believe the answer is:

arsenic

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'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" .)

I've seen this clue in The Hindu.
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