Tirade follows fresh, backsliding justification (7)
I believe the answer is:
warrant
'justification' is the definition.
'tirade follows fresh backsliding' is the wordplay.
'tirade' becomes 'rant' (both can mean an angry outburst).
'follows' means one lot of letters go next to another.
'fresh' becomes 'raw' (I've seen this in another clue).
'backsliding' shows that the letters should be reversed in order.
'raw' back-to-front is 'war'.
'rant' after 'war' is 'WARRANT'.
(Other definitions for warrant that I've seen before include "Document giving authority" , "Call for" , "Merit, justify" , "Guarantee" , "Guaranty - authorisation to make an arrest" .)