Poison sarnie upset Conservative (7)
I believe the answer is:
arsenic
'poison' is the definition.
(thesaurus)
'sarnie upset conservative' is the wordplay.
'upset' indicates an anagram.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'sarnie' with letters rearranged gives 'arseni'.
'arseni'+'c'='ARSENIC'
(Other definitions for arsenic that I've seen before include "Poisons in scare" , ", it goes with old lace" , "Element No 33" , "As briefly identified" , "Toxic metallic element used as rat poison" .)