Major no-no (5)
I believe the answer is:
taboo
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">major no-no</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for taboo that I've seen before include "Forbidden by social custom" , "A ban or prohibition of superstitious kind" , "Prohibited, sacred" , "No-no" , "ruled out" .)