Substance in tobacco (8)
I believe the answer is:
nicotine
'substance in tobacco' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for nicotine that I've seen before include "Material found in tobacco used as an insecticide" , "Hooker" , "Active constituent in tobacco" , "Once in it, strangely, one smokes it" , "Highly-addictive alkaloid" .)