Brief exchange about teacher from the south (9)
I believe the answer is:
barrister
'brief' is the definition.
(I've seen this before)
'exchange about teacher from the south' is the wordplay.
'exchange' becomes 'barter' (barter is a kind of exchange).
'about' indicates putting letters inside.
'teacher' becomes 'sir' (way of addressing a male teacher).
'from the south' says the letters should be written in reverse (in a down clue, letters go up).
'sir' in reverse letter order is 'ris'.
'barter' placed around 'ris' is 'BARRISTER'.
(Other definitions for barrister that I've seen before include "Legal representative" , "Legal advocate, must be briefed" , "One taking brief" , "Court lawyer" , "American lawyer" .)