Justifies tirades following hostilities (8)
I believe the answer is:
warrants
'justifies' is the definition.
('warrant' can be a synonym of 'justify')
'tirades following hostilities' is the wordplay.
'tirades' becomes 'rants' (I've seen this in other clues).
'following' is a charade indicator (letters next to each other).
'hostilities' becomes 'war' (I've seen this before).
'rants' put after 'war' is 'WARRANTS'.
(Other definitions for warrants that I've seen before include "Calls for - authorisations" , "They authorise the police to make arrests" , "Rats warn about documents authorising arrest or search" , "Written authorisations as for arrest" , "'Written legal authorisations, to make arrests say (8)'" .)