Poisonous plant made nark convulsive (8)
I believe the answer is:
mandrake
'poisonous plant' is the definition.
(plant of the nightshade family)
'made nark convulsive' is the wordplay.
'made nark' anagrams to 'MANDRAKE'.
But, I am unsure how the anagram is indicated.
Can you help me to learn more?
(Other definitions for mandrake that I've seen before include "Poisonous plant or cartoon magician" , "Plant once believed to shriek when pulled up" , "Root used in medicine" , "Poisonous plant with large forked root" , "Herbal root" .)