Lawyer working on treaty (8)
I believe the answer is:
attorney
'lawyer' is the definition.
(I know that attorney is a type of lawyer)
'working on treaty' is the wordplay.
'working' indicates anagramming the letters.
'on'+'treaty'='ontreaty'
'ontreaty' with letters rearranged gives 'ATTORNEY'.
(Other definitions for attorney that I've seen before include "Advocate" , "No treaty (anag.)" , "One legally acting for another" , "brief" , "One can help in case" .)