Forswear (6)
I believe the answer is:
abjure
'forswear' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for abjure that I've seen before include "Repudiate - renounce on oath" , "Renounce, forswear" , "(Solemnly) give up" , "Reject" , "Disavow" .)