Toxic element (7)
I believe the answer is:
arsenic
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">toxic element</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)