Open grassy area (3)
I believe the answer is:
lea
'open grassy area' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lea that I've seen before include "Open grassy land; river" , "pastoral setting" , "Meadowland" , "Tract of pastureland" , "''The lowing herd winds slowly o'er the ... '' (Gray)" .)