Grassland beyond river (7)
I believe the answer is:
pasture
'grassland' is the definition.
(pasture is a kind of grassland)
'beyond river' is the wordplay.
'beyond' becomes 'past' (I've seen this before).
'river' becomes 'ure' (I have seen 'River of Wensleydale ' mean 'ure' so perhaps 'river' could also mean 'ure').
'past'+'ure'='PASTURE'
(Other definitions for pasture that I've seen before include "Grassland" , "Meadowland" , "Grass for grazing" , "Meadow" , "Field suitable for grazing" .)