Pledge out of loathing (4)
I believe the answer is:
oath
'pledge' is the definition.
(I've seen this before)
'out of loathing' is the wordplay.
'out of' indicates a hidden word.
'OATH' is hidden in the letters of 'loathing'.
(Other definitions for oath that I've seen before include "Curse or solemn word" , "A curse - or a pledge" , "Swearword or solemn promise" , "word intended?" , "What frustrated people say" .)