Sheriff, courtesy of grass, arresting old woman (6)
I believe the answer is:
lawman
'sheriff' is the definition.
(I've seen this before)
'grass arresting old woman' is the wordplay.
'grass' becomes 'lawn' (I've seen this before).
'arresting' indicates putting letters inside.
'old woman' becomes 'ma' (I've seen this in other clues).
'lawn' placed around 'ma' is 'LAWMAN'.
'courtesy of' acts as a link.
(Other definitions for lawman that I've seen before include "Cop" , "Lex?" , "(In the US) a sheriff, for example" .)