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