Poison sarnie upset Charlie (7)
I believe the answer is:
arsenic
'poison' is the definition.
(I've seen this before)
'sarnie upset charlie' is the wordplay.
'upset' is an anagram indicator.
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'sarnie' is an anagram of 'arseni'.
'arseni'+'c'='ARSENIC'
(Other definitions for arsenic that I've seen before include "Poisons in scare" , "Poison found in races" , "-- and Old Lace (play)" , ", it goes with old lace" , "Ratsbane" .)