A treeless Arctic plain (6)
I believe the answer is:
tundra
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">a treeless arctic plain</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for tundra that I've seen before include "Vast treeless plane" , "Vast level Arctic region with frozen subsoil" , "Arctic plain with permafrost" , "wasteland" , "Sub-polar treeless zone" .)