Advocate working on treaty (8)
I believe the answer is:
attorney
'advocate' is the definition.
(thesaurus)
'working on treaty' is the wordplay.
'working' indicates anagramming the letters.
'on'+'treaty'='ontreaty'
'ontreaty' is an anagram of 'ATTORNEY'.
(Other definitions for attorney that I've seen before include "US lawyer" , "No treaty (anag.)" , "Solicitor or barrister" , "Advocate" , "One can help in case" .)