Back-of-the-book list (7)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">back-of-the-book list</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for addenda that I've seen before include "Extra items put in book" , "Extras" , "supplement" , "Textual supplements" , "number of supplements" .)