Bit at end of book that might be taken out? (8)
I believe the answer is:
appendix
This clue is a double definition.
'bit at end of book' is the first definition.
(section added to a book)
'that might be taken out?' is the second definition.
(the appendix can be removed from the body)
(Other definitions for appendix that I've seen before include "Extra section" , "extra pages" , "has no known use" , "Section at end of a book" , "Writer's addition - may be taken out in the theatre!" .)