Poison scare broadcast across Northern Ireland (7)
I believe the answer is:
arsenic
'poison' is the definition.
(thesaurus)
'scare broadcast across northern ireland' is the wordplay.
'broadcast' is an anagram indicator.
'across' means one lot of letters goes inside another.
'northern ireland' becomes 'ni'.
'scare' with letters rearranged gives 'arsec'.
'arsec' enclosing 'ni' is 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "Can rise result from poison?" , "Toxic metallic element used as rat poison" , "Poison used in pesticides and weedkillers" , "Ratsbane" , "Element with many poisonous compounds" .)