Poison involved in scare (7)
I believe the answer is:
arsenic
'poison' is the definition.
(arsenic can be used as a poison)
'involved in scare' is the wordplay.
'involved' indicates an anagram (involved can mean confusing or complex).
'in'+'scare'='inscare'
'inscare' anagrammed gives 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "-- and Old Lace (play)" , "Famous stage comedy ''. . . . . . . and Old Lace''" , "Poison found in races" , "As briefly identified" , "Brittle grey-green semi-metal, atomic number 33" .)