Forbidden by custom (5)
I believe the answer is:
taboo
'forbidden by custom' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for taboo that I've seen before include "No-no" , "Ritually forbidden" , "Prohibited as unclean or holy" , "Socially prohibited" , "Not to be touched or used, banned for religious reasons" .)