Add as a supplement (6)
I believe the answer is:
append
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">add as a supplement</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for append that I've seen before include "Add, annex" , "Affix to the end" , "Add (to end of text)" , "Napped (anag)" , "Add (at the back of a document)" .)