To this matter (6)
I believe the answer is:
hereto
'to this matter' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)