Begin filming battle (6)
I believe the answer is:
action
'battle' is the definition.
(military action)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for action that I've seen before include "The process of doing something" , "Jarndyce and Jarndyce, say" , "Combat" , "Performance" , "Battle (in the law courts?)" .)