Bill interrupted by Greek character, one who writes abject verses (9)
I believe the answer is:
poetaster
'one who writes abject verses' is the definition.
(someone who writes bad poetry)
'bill interrupted by greek character' is the wordplay.
'bill' becomes 'poster' (synonyms).
'interrupted by' means one lot of letters goes inside another.
'greek character' becomes 'eta' (letter from the Greek alphabet).
'poster' placed around 'eta' is 'POETASTER'.
(Other definitions for poetaster that I've seen before include "He writes inferior verse" , "Poor writer of verse" , "William McGonagall" , "Writer of doggerel" , "Inferior writer" .)