Atrocious teller being argumentative (11)
I believe the answer is:
belligerent
'argumentative' is the definition.
(thesaurus)
'atrocious teller being' is the wordplay.
'atrocious' indicates an anagram.
'teller'+'being'='tellerbeing'
'tellerbeing' is an anagram of 'BELLIGERENT'.
(Other definitions for belligerent that I've seen before include "fighting" , "Hostile, aggressive" , "Combative" , "Warlike" , "hawk" .)