Brief film about outbreak of war (6)
I believe the answer is:
lawyer
'brief' is the definition.
(slang term for a lawyer)
'film about outbreak of war' is the wordplay.
'film' becomes 'layer' (film can mean a thin layer of something).
'about' indicates putting letters inside.
'outbreak of' says to take the initial letters.
The initial letter of 'war' is 'w'.
'layer' placed around 'w' is 'LAWYER'.
(Other definitions for lawyer that I've seen before include "Legal representative" , "Attorney in the US" , "Legal professional" , "court worker" , "One gives legal advice" .)