Call to join total battle (7)
I believe the answer is:
summons
'call to join' is the definition.
(I've seen this before)
'total battle' is the wordplay.
'total' becomes 'sum' (synonyms).
'battle' becomes 'mons' (Battle of Mons, WW1 battle).
'sum'+'mons'='SUMMONS'
(Other definitions for summons that I've seen before include "orders" , "Call (to court)" , "Call to join" , "Order to appear in law court" , "Legal order" .)