Leaving southern Frenchman heading west (6)
I believe the answer is:
egress
'leaving' is the definition.
(I know that leaving can be written as egress)
'southern frenchman heading west' is the wordplay.
'southern' becomes 's' (abbreviation).
'frenchman' becomes 'serge' (I've seen this before).
'heading west' says the letters should be written backwards.
's'+'serge'='sserge'
'sserge' back-to-front is 'EGRESS'.
(Other definitions for egress that I've seen before include "The act of leaving a building" , "Come out" , "Way out; going out" , "Way out of serges" , "Way out, departure" .)