Arctic plain (6)
I believe the answer is:
tundra
'arctic plain' is the definition.
(type of geographical region)
This is all the clue.
(Other definitions for tundra that I've seen before include "Treeless northern region" , "Area of permanently frozen subsoil" , "Area of permafrost" , "where you might see caribou" , "Vast flat treeless zone" .)