Substance in tobacco (8)
I believe the answer is:
nicotine
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">substance in tobacco</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 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" .)