Figure straddling single bed is poison (8)
I believe the answer is:
nicotine
'is poison' is the definition.
(I've seen this before)
'figure straddling single bed' is the wordplay.
'figure' becomes 'nine' (I've seen this in other clues**).
'straddling' indicates putting letters inside (straddle can mean to lie on both sides of).
'single' becomes 'i' (one).
'bed' becomes 'cot' (cot is a kind of bed).
'i'+'cot'='icot'
'nine' enclosing 'icot' is 'NICOTINE'.
(Other definitions for nicotine that I've seen before include "Poison not in ice" , "Tobacco alkaloid" , "Toxic substance in tobacco" , "Drug often smoked" , "Drug in tobacco" .)