Swearword; pledge (4)
I believe the answer is:
oath
I believe this clue is a double definition.
'swearword' is the first definition.
(synonyms)
'pledge' is the second definition.
(I've seen this in another clue)
(Other definitions for oath that I've seen before include "A curse - or a pledge" , "It's taken in court" , "swearing" , "'Profane, or solemn expression (4)'" , "Solemn promise to tell the truth or be loyal" .)