Problem with battle order (7)
I believe the answer is:
summons
'order' is the definition.
(I know that summons is a type of order)
'problem with battle' is the wordplay.
'problem' becomes 'sum' (arithmetic problem).
'with' means one lot of letters go next to another.
'battle' becomes 'mons' (I've seen this before).
'sum'+'mons'='SUMMONS'
(Other definitions for summons that I've seen before include "Order to attend court" , "Call (to court)" , "Calls to appear" , "orders" , "Legal order" .)