Two rounds added to bill — that's unacceptable (5)
I believe the answer is:
taboo
'unacceptable' 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' put after 'tab' is 'TABOO'.
'that's' is the link.
(Other definitions for taboo that I've seen before include "Prohibited; ban" , "Not allowed, forbidden, proscribed" , "No-no" , "Ritually forbidden" , "Inhibition based on social custom" .)