Toxic metalloid element (7)
I believe the answer is:
arsenic
'toxic metalloid element' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for arsenic that I've seen before include "Element with many poisonous compounds" , "Ratsbane" , "Toxic metallic element used as rat poison" , "Poison found in races" , "Element No 33" .)