Poisonous element (7)
I believe the answer is:
arsenic
'poisonous element' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for arsenic that I've seen before include "Toxic metallic element used as rat poison" , "As briefly identified" , "Element with many poisonous compounds" , "Poison used in pesticides and weedkillers" , "Powerful poison" .)