Swear solution contains sulphur (5)
I believe the answer is:
curse
'swear' is the definition.
(I know that swear can be written as curse)
'solution contains sulphur' is the wordplay.
'solution' becomes 'cure' (cure can mean a solution or treatment).
'contains' indicates putting letters inside.
'sulphur' becomes 'S' (synonyms).
'cure' going around 's' is 'CURSE'.
(Other definitions for curse that I've seen before include "Anathema" , "Wish evil upon" , "Evil spell - oath" , "Blasphemy" , "Blaspheme; damn" .)