Aggressive teller being awkward (11)
I believe the answer is:
belligerent
'aggressive' is the definition.
(synonyms)
'teller being awkward' is the wordplay.
'awkward' indicates anagramming the letters.
'teller'+'being'='tellerbeing'
'tellerbeing' anagrammed gives 'BELLIGERENT'.
(Other definitions for belligerent that I've seen before include "hawk" , "fighting" , "Aggressive - it'll be green (anag)" , "Combative" , "Hostile, aggressive" .)