Not on bill: two rounds (5)
I believe the answer is:
taboo
'not on' is the definition.
I can't judge whether this definition defines the answer.
'bill two rounds' is the wordplay.
'two rounds' means two substitutions for 'round'.
'bill' becomes 'tab' (a tab is a bill at a bar).
'round' becomes 'o' (round can mean a round object).
'round' becomes 'o' (round can mean a round object).
'tab'+'o'+'o'='TABOO'
Can you help me to learn more?
(Other definitions for taboo that I've seen before include "Prohibited as unclean or holy" , "Ritual prohibition" , "Ritually forbidden" , "Not to be touched or used, banned for religious reasons" , "(Irrational) prohibition" .)