Not to be mentioned (5)
I believe the answer is:
taboo
'not to be mentioned' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for taboo that I've seen before include "Not to be touched or used, banned for religious reasons" , "Strictly forbidden" , "Anathema" , "Prohibited by social custom" , "(Something) forbidden" .)