Curse out of loathing (4)
I believe the answer is:
oath
'curse' is the definition.
(synonyms)
'of loathing' is the wordplay.
'of' indicates a hidden word.
'OATH' is hidden within 'loathing'.
'out' acts as a 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" .)