Remedy includes small oath (5)
I believe the answer is:
curse
'oath' is the definition.
(I know that oath can be written as curse)
'remedy includes small' is the wordplay.
'remedy' becomes 'cure' (synonyms).
'includes' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cure' placed around 's' is 'CURSE'.
(Other definitions for curse that I've seen before include "Cuss" , "Cause of harm or misery" , "Wish harm on" , "Evil spell - oath" , "use bad language" .)