In this matter (6)
I believe the answer is:
hereto
'in this matter' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hereto that I've seen before include "To this document, in legal jargon" , "To this writing or document, in legalese" , "To this place (law)" , "(Appended) to this document" , "To this place or time" .)