Figure hosting company knocked back vermouth — it’s addictive stuff (8)
I believe the answer is:
nicotine
'addictive stuff' is the definition.
(I've seen this in another clue)
'figure hosting company knocked back vermouth' is the wordplay.
'figure' becomes 'nine' (a number of figure).
'hosting' is an insertion indicator (inserted letters are welcomed in or hosted).
'company' becomes 'co' (abbreviation).
'knocked back' says the letters should be written backwards.
'vermouth' becomes 'it' (short for 'Italian vermouth' as in 'gin and it').
'it' written backwards gives 'ti'.
'co'+'ti'='coti'
'nine' enclosing 'coti' is 'NICOTINE'.
'it's' acts as a link.
(Other definitions for nicotine that I've seen before include "Tobacco alkaloid" , "Component of tobacco" , "Active constituent in tobacco" , "Toxic substance in tobacco" , "Tobacco poison found in notice" .)