Order to begin shooting in battle (6)
I believe the answer is:
action
I believe this is a double definition.
'order to begin shooting' is the first definition.
(as in 'action' on a film set)
'battle' is the second definition.
(military action)
'in' is the link.
(Other definitions for action that I've seen before include "Law suit" , "Operation - lawsuit" , "skirmish" , "The mechanism of, say, a watch" , "Deed; armed conflict" .)