Add or attach (5)
I believe the answer is:
annex
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">add</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">attach</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for annex that I've seen before include "Take by force" , "An addition that extends a main building" , "Addition to a document" , "Append" , "Add by conquest or occupation" .)