Grassland first not last (3)
I believe the answer is:
lea
'grassland' is the definition.
(lea is a kind of grassland)
'first not last' is the wordplay.
'first' becomes 'lead' (lead can mean leading or primary).
'not last' means to remove the last letter.
'lead' with its final letter taken away is 'LEA'.
(Other definitions for lea that I've seen before include "'The lowing herd wind slowly o'er the . . .'" , "Open pasture" , "River flowing through London" , "grazing area" , "Pasture, London river" .)