Addition to a book (8)
I believe the answer is:
appendix
'addition to a book' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for appendix that I've seen before include "Extra section" , "Table of subsidiary matter - vestigial organ" , "Addendum - vestigial pouch in human intestines" , "Section at end of a book" , "Useless vestigial abdominal pouch" .)