Extra item at end of book (8)
I believe the answer is:
addendum
'extra item at end of book' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for addendum that I've seen before include "Supplement, appendix" , "PS" , "Extra item added to book or text" , "Supplementary text tacked on to the end" , "Extra item added at end of book" .)