To this matter (6)
I believe the answer is:
hereto
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">to this matter</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for hereto that I've seen before include "Attached to this document, legal term" , "In this direction" , "To this document, in legal jargon" , "To this point or time (6" , "To this place or time" .)