Swear by remedy containing sulphur (5)
I believe the answer is:
curse
'swear' is the definition.
('curse' can be a synonym of 'swear')
'remedy containing sulphur' is the wordplay.
'remedy' becomes 'cure' ('cure' can be a synonym of 'remedy').
'containing' is an insertion indicator.
'sulphur' becomes 's' ('S' can be a synonym of 'sulphur').
'cure' enclosing 's' is 'CURSE'.
'by' is the link.
(Other definitions for curse that I've seen before include "Cuss" , "Wish harm on" , "Cause of harm or misery" , "Imprecation or oath" , "Wish evil upon" .)