Guarantees, when the battle rages (8)
I believe the answer is:
warrants
'guarantees' is the definition.
('warrant' can be a synonym of 'guarantee')
'the battle rages' is the wordplay.
'the battle' becomes 'war' (armed conflict).
'rages' becomes 'rants' (I can't explain this - if you can you should believe this answer much more).
'war'+'rants'='WARRANTS'
'when' is the link.
Can you help me to learn more?
(Other definitions for warrants that I've seen before include "Rats warn about documents authorising arrest or search" , "Justifies" , "'Written legal authorisations, to make arrests say (8)'" , "Calls for - authorisations" , "Written authorisations as for arrest" .)