Bitterness about British holy book (5)
I believe the answer is:
bible
'holy book' is the definition.
(I've seen this before)
'bitterness about british' is the wordplay.
'bitterness' becomes 'bile' (I've seen this in other clues).
'about' indicates putting letters inside.
'british' becomes 'b' (abbreviation e.g. in BBC).
'bile' enclosing 'b' is 'BIBLE'.
(Other definitions for bible that I've seen before include "Book regarded as the ultimate authority on its subject" , "Authoritative source" , "The Christian or Jewish scriptures" , "Book for someone swearing?" , "reliable work" .)