Plague remedy includes sulphur (5)
I believe the answer is:
curse
'plague' is the definition.
(thesaurus)
'remedy includes sulphur' is the wordplay.
'remedy' becomes 'cure' ('cure' can be a synonym of 'remedy').
'includes' means one lot of letters goes inside another.
'sulphur' becomes 's' ('S' can be a synonym of 'sulphur').
'cure' going around 's' is 'CURSE'.
(Other definitions for curse that I've seen before include "Imprecate" , "Blasphemy" , "Anathema" , "It's bad spelling" , "Invoke evil upon" .)