Authorisation of combat leading to diatribe (7)
I believe the answer is:
warrant
'authorisation' is the definition.
(I've seen this before)
'combat leading to diatribe' is the wordplay.
'combat' becomes 'war' (I've seen this before).
'leading to' is a charade indicator (letters next to each other) (some letters go before or lead others).
'diatribe' becomes 'rant' (I've seen this in another clue).
'war'+'rant'='WARRANT'
'of' is the link.
(Other definitions for warrant that I've seen before include "Permit for arrest, search of premises, etc" , "Merit, justify" , "Guarantee" , "Authorise, justify" , "Guaranty - authorisation to make an arrest" .)