Poisonous substance in tobacco (8)
I believe the answer is:
nicotine
'poisonous substance in tobacco' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nicotine that I've seen before include "Not nice, I change to cigarette substance" , "Poison not in ice" , "Tobacco poison found in notice" , "Material found in tobacco used as an insecticide" , "Drug in tobacco" .)