Treeless grassy plain (7)
I believe the answer is:
prairie
'treeless grassy plain' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for prairie that I've seen before include "Open grassland in the USA" , "Extensive US grassland" , "Large tract of grassland in N America" , "Grasslands" , "Schooners once crossed it" .)