Addition to a document (5)
I believe the answer is:
annex
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">addition to a document</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for annex that I've seen before include "Add to one's territory" , "Attach after conquest" , "Add by conquest or occupation" , "Extension to a main building" , "Attach(ment)" .)