Forbidden in polite society (5)
I believe the answer is:
taboo
'forbidden in polite society' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for taboo that I've seen before include "Prohibited by social custom" , "Not allowed, forbidden, proscribed" , "Prohibited by religion or custom" , "Beyond the pale" , "Strictly forbidden" .)