Toxic element (7)
I believe the answer is:
arsenic
'toxic element' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for arsenic that I've seen before include "Powerful poison" , "Ratsbane" , "Can rise result from poison?" , "-- and Old Lace (play)" , "Element with many poisonous compounds" .)