Supplement, appendix (8)
I believe the answer is:
addendum
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">supplement</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">appendix</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for addendum that I've seen before include "Something appended as to end of book" , "Textual matter added on" , "Text added at the end" , "Supplement to a book" , "Dud ad-men dealt with in end of book" .)