Poison spilt in case? About right (7)
I believe the answer is:
arsenic
'poison' is the definition.
(arsenic can be used as a poison)
'spilt in case? about right' is the wordplay.
'spilt' indicates an anagram.
'about' indicates putting letters inside.
'right' becomes 'r' (common abbreviation).
'in'+'case'='incase'
'incase' anagrammed gives 'asenic'.
'asenic' enclosing 'r' is 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "Element with many poisonous compounds" , "Toxic metallic element used as rat poison" , "Element No 33" , ", it goes with old lace" , "Can rise result from poison?" .)