Scare in spilling poison (7)
I believe the answer is:
arsenic
'poison' is the definition.
(thesaurus)
'scare in spilling' is the wordplay.
'spilling' indicates an anagram.
'scare'+'in'='scarein'
'scarein' anagrammed gives 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "Brittle grey-green semi-metal, atomic number 33" , "Toxic metalloid element" , "Famous stage comedy ''. . . . . . . and Old Lace''" , "As briefly identified" , "Powerful poison" .)