Element involved in scare (7)
I believe the answer is:
arsenic
'element' is the definition.
(I know that arsenic is a metalloid)
'involved in scare' is the wordplay.
'involved' indicates anagramming the letters (involved can mean confusing or complex).
'in'+'scare'='inscare'
'inscare' with letters rearranged gives 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "Element with many poisonous compounds" , "killer" , "Poison used in pesticides and weedkillers" , "Toxic metallic element used as rat poison" , ", it goes with old lace" .)