Bill, the fellow being a pain (4)
I believe the answer is:
ache
'being a pain' is the definition.
(I've seen this before)
'bill the fellow' is the wordplay.
'bill' becomes 'ac' (short for account).
'the fellow' becomes 'he' (pronoun for a man).
'ac'+'he'='ACHE'
(Other definitions for ache that I've seen before include "painful" , "Nagging pain" , "suffering" , "Shakespearean character" , "Pine" .)