Conflict confined by mass defence (7)
I believe the answer is:
bulwark
'defence' is the definition.
(bulwark is a kind of defence)
'conflict confined by mass' is the wordplay.
'conflict' becomes 'war' (war is a kind of conflict).
'confined by' is an insertion indicator.
'mass' becomes 'bulk' (bulk can mean a mass of something).
'war' inserted inside 'bulk' is 'BULWARK'.
(Other definitions for bulwark that I've seen before include "Buttress" , "Defensive wall" , "defence" , "Fortification - side of ship above deck" , "Rampart" .)