Two rounds added to bill? That's ruled out (5)
I believe the answer is:
taboo
'ruled out' is the definition.
(I've seen this before)
'two rounds added to bill?' is the wordplay.
'two rounds' becomes 'oo' (two round letters).
'added to' is a charade indicator (letters next to each other).
'bill?' becomes 'tab' (a tab is a bill at a bar).
'oo' after 'tab' is 'TABOO'.
'that's' acts as a link.
(Other definitions for taboo that I've seen before include "ruled out" , "Prohibited by social custom" , "Socially prohibited" , "A ban or prohibition of superstitious kind" , "Ritually forbidden" .)