Sarnie prepared first contained poison (7)
I believe the answer is:
arsenic
'poison' is the definition.
(arsenic can be used as a poison)
'sarnie prepared first contained' is the wordplay.
'prepared' indicates anagramming the letters.
'first contained' becomes 'c' (1st letter of 'contained').
'sarnie' anagrammed gives 'arseni'.
'arseni'+'c'='ARSENIC'
(Other definitions for arsenic that I've seen before include "As briefly identified" , "Toxic metalloid element" , ", it goes with old lace" , "-- and Old Lace (play)" , "killer" .)