Start to shoot in army engagement (6)
I believe the answer is:
action
'engagement' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for action that I've seen before include "Military engagement" , "war?" , "Thing done; armed conflict" , "Lawsuit; something done" , "A judicial proceeding brought by one party against another" .)