Involve in silly scare about a poison (7)
I believe the answer is:
arsenic
'a poison' is the definition.
(thesaurus)
'involve in silly scare about' is the wordplay.
'involve' indicates an anagram.
'silly' is an anagram indicator.
'about' indicates putting letters inside.
'scare' with letters rearranged gives 'arsec'.
'in' anagrammed gives 'ni'.
'ni' put into 'arsec' is 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "Brittle grey-green semi-metal, atomic number 33" , "As briefly identified" , "Poison used in pesticides and weedkillers" , "Ratsbane" , "killer" .)