Unmentionable (5)
I believe the answer is:
taboo
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unmentionable</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)