Gentleman sent back in exchange for lawyer (9)
I believe the answer is:
barrister
'for lawyer' is the definition.
(barrister is a kind of lawyer)
'gentleman sent back in exchange' is the wordplay.
'gentleman' becomes 'sir' (a man may be addressed as sir).
'sent back' says the letters should be written backwards.
'in' indicates putting letters inside.
'exchange' becomes 'barter' (barter is a kind of exchange).
'sir' reversed gives 'ris'.
'ris' put into 'barter' is 'BARRISTER'.
(Other definitions for barrister that I've seen before include "member of an inn" , "Court lawyer" , "Legal representative" , "Rarest rib for the eminent lawyer" , "American lawyer" .)