Goddess encountered in grass, rolling over (7)
I believe the answer is:
demeter
'goddess' is the definition.
(thesaurus)
'encountered in grass rolling over' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'in' means one lot of letters goes inside another.
'grass' becomes 'reed' (I've seen this before**).
'rolling over' is a reversal indicator.
'reed' back-to-front is 'deer'.
'met' placed into 'deer' is 'DEMETER'.
(Other definitions for demeter that I've seen before include "Gr. goddess of agriculture, mother of Persephone" , "Greek goddess of fertility - metered (anag)" , "Olympian" , "Divine female" , "Ancient Greek deity of fertility" .)