Poison, i.e. candy concoction (7)
I believe the answer is:
cyanide
'poison' is the definition.
(cyanide is poisonous)
'i e candy concoction' is the wordplay.
'concoction' is an anagram indicator.
'i'+'e'+'candy'='iecandy'
'iecandy' with letters rearranged gives 'CYANIDE'.
(Other definitions for cyanide that I've seen before include "Toxic salt" , "Poisonous compound" , "Extremely poisonous salt" , "killer" , "Highly toxic substance" .)