Brief film about onset of war (6)
I believe the answer is:
lawyer
'brief' is the definition.
(I've seen this before)
'film about onset of war' is the wordplay.
'film' becomes 'layer' (I've seen this before).
'about' means one lot of letters goes inside another.
'onset 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 "court worker" , "Attorney in the US" , "Solicitor, for example" , ""Let's kill all the --s" (Henry VI/2)" , "Legal practitioner" .)