Additional, ancillary (13)
I believe the answer is:
supplementary
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">additional</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">ancillary</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for supplementary that I've seen before include "Extra" , "Added to complete or make up a deficiency" , "Ancillary" , "Additional" .)