"Die, Bart!", I horribly harangue (8)
I believe the answer is:
diatribe
'harangue' is the definition.
(thesaurus)
'die bart i horribly' is the wordplay.
'horribly' indicates an anagram.
'die'+'bart'+'i'='diebarti'
'diebarti' with letters rearranged gives 'DIATRIBE'.
(Other definitions for diatribe that I've seen before include "provoking verbal onslaught" , "Lengthy harangue" , "critical speech" , "Stream of invective" , "Bitter words" .)