Supplement at the end of a book (8)
I believe the answer is:
appendix
'supplement at the end of a book' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for appendix that I've seen before include "Table of subsidiary matter - vestigial organ" , "It comes last" , "extra pages" , "Useless vestigial abdominal pouch" , "Subsidiary section" .)