Poison scare in mess (7)
I believe the answer is:
arsenic
'poison' is the definition.
(arsenic can be used as a poison)
'scare in mess' is the wordplay.
'mess' indicates anagramming the letters.
'scare'+'in'='scarein'
'scarein' with letters rearranged gives 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "killer" , "Element with many poisonous compounds" , "Poison found in races" , "Element No 33" , "Toxic metallic element used as rat poison" .)