Swear an oath (5)
I believe the answer is:
curse
This clue is a double definition.
'swear' is the first definition.
(I know that swear can be written as curse)
'an oath' is the second definition.
(I know that oath can be written as curse)
(Other definitions for curse that I've seen before include "Excommunication" , "Cause of harm or misery" , "Profane or obscene expression" , "Profanity" , "Anathema" .)