Open area of grassy land (3)
I believe the answer is:
lea
'open area of grassy land' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lea that I've seen before include "Grass area" , "River of east London" , "Meadowland" , "Open pasture" , "''The lowing herd winds slowly o'er the ... '' (Gray)" .)