Poison sarnie ruined Conservative (7)
I believe the answer is:
arsenic
'poison' is the definition.
(arsenic can be used as a poison)
'sarnie ruined conservative' is the wordplay.
'ruined' indicates anagramming the letters.
'conservative' becomes 'C' (UK political abbreviation).
'sarnie' with letters rearranged gives 'arseni'.
'arseni'+'c'='ARSENIC'
(Other definitions for arsenic that I've seen before include ", it goes with old lace" , "Poison used in pesticides and weedkillers" , "Poison found in races" , "Ratsbane" , "Element No 33" .)