Loathsome born hater getting agitated (9)
I believe the answer is:
abhorrent
'loathsome' is the definition.
(thesaurus)
'born hater getting agitated' is the wordplay.
'getting agitated' is an anagram indicator.
'born'+'hater'='bornhater'
'bornhater' with letters rearranged gives 'ABHORRENT'.
(Other definitions for abhorrent that I've seen before include "Hated" , "hateful" , "horrid" , "execrable" , "Inspiring disgust and loathing" .)