Regarding this matter
I believe the answer is:
hereto
'regarding 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 "object" , "The ore (anag)" , "(Appended) to this document" , "Attached to this document, legal term" , "To this point or time (6" .)