Crossword Genius

Hazardous substance from canister, unstable when temperature drops (7)

Ross

I believe the answer is:

arsenic

'hazardous substance' is the definition.
(arsenic is poisonous)

I can't explain the rest of the clue.

(Other definitions for arsenic that I've seen before include "Poison used in pesticides and weedkillers" , "Element No 33" , "Toxic metalloid element" , ", it goes with old lace" , "Ratsbane" .)

I've seen this clue in The Guardian.
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