A right harangue - and thorough! (6)
I believe the answer is:
arrant
'thorough' is the definition.
(thesaurus)
'a right harangue' is the wordplay.
'a right' becomes 'ar' (I've seen this in other clues).
'harangue' becomes 'rant' ('rant' can be a synonym of 'harangue').
'ar'+'rant'='ARRANT'
'and' acts as a link.
(Other definitions for arrant that I've seen before include "Extreme and utter, as nonsense may be" , "Utter and absolute, maybe nonsense" , "Blatant" , "Downright - notorious" , "Complete (nonsense)" .)