Poisonous weed (7)
I believe the answer is:
hemlock
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">poisonous weed</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 hemlock that I've seen before include "preparation to kill" , "Poison killing Socrates" , "Poisonous drug" , "Socratic poison" , "Poisonous plant" .)