That woman on grass is pretty (6)
I believe the answer is:
rather
'is pretty' is the definition.
(I've seen this before)
'that woman on grass' is the wordplay.
'that woman' becomes 'her' ('her' refers to a woman).
'on' means one lot of letters go next to another.
'grass' becomes 'rat' (both can mean a snitch or informer).
'her' after 'rat' is 'RATHER'.
(Other definitions for rather that I've seen before include "Somewhat; more willingly" , "On the contrary or to some extent" , "More precisely; instead of" , "To a limited degree" , "Pretty" .)