A right diatribe is extreme! (6)
I believe the answer is:
arrant
'extreme' is the definition.
(I've seen this in another clue)
'a right diatribe' is the wordplay.
'a right' becomes 'ar' (I've seen this in other clues).
'diatribe' becomes 'rant' (I've seen this in other clues).
'ar'+'rant'='ARRANT'
'is' acts as a link.
(Other definitions for arrant that I've seen before include "Utter; downright" , "'Complete, stark (6)'" , "Out-and-out, notorious" , "Downright - notorious" , "Blatant" .)