To occupy oneself go into battle (6)
I believe the answer is:
engage
'to occupy' is the definition.
('engage' can be a synonym of 'occupy')
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for engage that I've seen before include "start fighting" , "Recruit" , "Take into service or commit to marriage" , "Take on - reserve - promise" , "Secure" .)