Bill loves forbidden subject (5)
I believe the answer is:
taboo
'forbidden subject' is the definition.
(something that can't be discussed)
'bill loves' is the wordplay.
'bill' becomes 'tab' (a tab is a bill at a bar).
'loves' becomes 'oo' (two O's - 'love' means zero in tennis).
'tab'+'oo'='TABOO'
(Other definitions for taboo that I've seen before include "Not to be touched or used, banned for religious reasons" , "No-no" , "Prohibited; ban" , "Socially prohibited" , "Strictly forbidden" .)