Noblemen poison wine (9)
I believe the answer is:
margraves
'noblemen' is the definition.
(margrave is a kind of nobleman)
'poison wine' is the wordplay.
'poison' becomes 'mar' (mar can mean to spoil or poison).
'wine' becomes 'Graves' (wine from the Graves region in France).
'mar'+'graves'='MARGRAVES'
(Other definitions for margraves that I've seen before include "aristocrats abroad" , "Noblemen" .)