Key rings prohibited (5)
I believe the answer is:
taboo
'prohibited' is the definition.
(I know that prohibited can be written as taboo)
'key rings' is the wordplay.
'key' becomes 'tab' (key on a computer keyboard).
'rings' becomes 'oo' (multiple circles).
'tab'+'oo'='TABOO'
(Other definitions for taboo that I've seen before include "Ritual prohibition" , "Forbidden by social custom" , "Not to be mentioned" , "A ban or prohibition of superstitious kind" , "unacceptable" .)