Fellow, just horrible (9)
I believe the answer is:
frightful
'horrible' is the definition.
(I know that horrible can be written as frightful)
'fellow just' is the wordplay.
'fellow' becomes 'F' (abbreviation - of a society etc.).
'just' becomes 'rightful' (similar in meaning).
'f'+'rightful'='FRIGHTFUL'
(Other definitions for frightful that I've seen before include "horrible" , "Appalling" , "nasty" , "savage" , "from hell" .)