Endless unfertile pasture (3)
I believe the answer is:
lea
'pasture' is the definition.
('lea' can be a synonym of 'pasture')
'endless unfertile' is the wordplay.
'endless' means to remove the last letter.
'unfertile' becomes 'lean'.
'lean' with its final letter taken away is 'LEA'.
(Other definitions for lea that I've seen before include "Open grassy land; river" , "Open country" , "Meadowland" , "Open arable land (poet.)" , "River of east London" .)