Prohibited a bishop entering as well (5)
I believe the answer is:
taboo
'prohibited' is the definition.
(I know that prohibited can be written as taboo)
'a bishop entering as well' is the wordplay.
'bishop' becomes 'B' (abbreviation for bishop in chess).
'entering' is an insertion indicator.
'as well' becomes 'too' (both can mean 'also').
'a'+'b'='ab'
'ab' inserted into 'too' is 'TABOO'.
(Other definitions for taboo that I've seen before include "(Irrational) prohibition" , "Ritual prohibition" , "Forbidden by social custom" , "Not allowed, forbidden, proscribed" , "No-no" .)