Curse out of loathing (4)
I believe the answer is:
oath
'curse' is the definition.
(synonyms)
'of loathing' is the wordplay.
'of' says the answer is hidden in the clue.
'OATH' can be found hidden inside 'loathing'.
'out' is the link.
(Other definitions for oath that I've seen before include "Swearword or solemn promise" , "Solemn vow" , "A curse or solemn declaration" , "Courtroom promise" , "blasphemy" .)