Social prohibition (5)
I believe the answer is:
taboo
'social prohibition' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for taboo that I've seen before include "Not to be touched or used, banned for religious reasons" , "A ban or prohibition of superstitious kind" , "Forbidden for social reasons" , "Not permitted" , "Socially prohibited" .)