Swear remedy involves sun (5)
I believe the answer is:
curse
'swear' is the definition.
('curse' can be a synonym of 'swear')
'remedy involves sun' is the wordplay.
'remedy' becomes 'cure' ('cure' can be a synonym of 'remedy').
'involves' is an insertion indicator.
'sun' becomes 's'.
'cure' going around 's' is 'CURSE'.
(Other definitions for curse that I've seen before include "Execrate" , "Profane or obscene expression" , "Imprecation or oath" , "Cuss" , "Profanity" .)