Key rings forbidden by custom (5)
I believe the answer is:
taboo
'forbidden by custom' is the definition.
(I've seen this before)
'key rings' is the wordplay.
'key' becomes 'tab' (key on a computer keyboard).
'rings' becomes 'oo' (two O's - O resembles a ring).
'tab'+'oo'='TABOO'
(Other definitions for taboo that I've seen before include "(Irrational) prohibition" , "Socially prohibited" , "Ritual prohibition" , "Prohibited as unclean or holy" , "Beyond the pale" .)