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