Major no-no (5)
I believe the answer is:
taboo
'major no-no' 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 "Forbidden by social custom" , "A ban or prohibition of superstitious kind" , "Prohibited, sacred" , "No-no" , "ruled out" .)