Legal expert upset gentleman in deal (9)
I believe the answer is:
barrister
'legal expert' is the definition.
(thesaurus)
'upset gentleman in deal' is the wordplay.
'upset' says the letters should be written in reverse.
'gentleman' becomes 'sir' (I've seen this before).
'in' indicates putting letters inside.
'deal' becomes 'barter'.
'sir' reversed gives 'ris'.
'ris' put inside 'barter' is 'BARRISTER'.
(Other definitions for barrister that I've seen before include "member of an inn" , "Lawyer for the higher courts" , "One represents clients in court" , "American lawyer" , "Legal advocate, must be briefed" .)