Terrible fellow you reportedly left after a week (5)
I believe the answer is:
awful
'terrible' is the definition.
(I know that terrible can be written as awful)
'fellow you reportedly left after a week' is the wordplay.
'fellow' becomes 'F' (abbreviation - of a society etc.).
'you reportedly' becomes 'U' (homophone of 'you').
'left' becomes 'l' (common abbreviation).
'after' is a charade indicator (letters next to each other).
'week' becomes 'w' (abbreviation).
'f'+'u'+'l'='ful'
'a'+'w'='aw'
'ful' put after 'aw' is 'AWFUL'.
(Other definitions for awful that I've seen before include "Atrocious, dire" , "Horrible" , "Fearsome" , "Not good at all" , "Very poor" .)