Treeless grassy plain (7)
I believe the answer is:
prairie
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">treeless grassy plain</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)