Poisonous weed (7)
I believe the answer is:
hemlock
'poisonous weed' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for hemlock that I've seen before include "preparation to kill" , "Poison killing Socrates" , "Poisonous drug" , "Socratic poison" , "Poisonous plant" .)