Plan military commander keeps close (6)
I believe the answer is:
agenda
'plan' is the definition.
(agenda is a kind of plan)
'military commander keeps close' is the wordplay.
'military commander' becomes 'aga' (I've seen this before).
'keeps' is an insertion indicator.
'close' becomes 'end' (I've seen this before).
'aga' enclosing 'end' is 'AGENDA'.
(Other definitions for agenda that I've seen before include "Timetable" , "List of items to be discussed" , "Meet to discuss this" , "List of items of business for committee" , "Plan for meeting" .)