Lawyer vacillating on treaty (8)
I believe the answer is:
attorney
'lawyer' is the definition.
(synonyms)
'vacillating on treaty' is the wordplay.
'vacillating' indicates anagramming the letters.
'on'+'treaty'='ontreaty'
'ontreaty' with letters rearranged gives 'ATTORNEY'.
(Other definitions for attorney that I've seen before include "Person legally authorised to act for another" , "Lawman" , "profession" , "Porter's first employer's occupation" , "Advocate" .)