Unmentionable (5)
I believe the answer is:
taboo
'unmentionable' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for taboo that I've seen before include "Subject to prohibition" , "Not allowed, forbidden, proscribed" , "Prohibited, sacred" , "(Something) forbidden" , "Forbidden by social custom" .)