Grassy expanse (3)
(Other definitions for prairie that I've seen before include "Tract of (US) grassland" , "Treeless grassy plain, in the old West perhaps" , "Broad rolling tract of land in the US" , "Cowboy song: ` `Bury me not on the lone .......' '" , "this oyster produces an egg" .)
'grassy expanse' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lea that I've seen before include "River flowing through London" , "authority for teachers" , "pastoral setting" , "Open arable land (poet.)" , "Field" .)