Part of the Old Testament, a book that's banned (5)
I believe the answer is:
taboo
'banned' is the definition.
(I've seen this in another clue)
'part of the old testament a book' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'TABOO' is hidden in the letters of 'old testament a book'.
'that's' acts as a link.
(Other definitions for taboo that I've seen before include "No-no" , "ruled out" , "Prohibited by religion or custom" , "A ban or prohibition of superstitious kind" , "Ritual prohibition" .)