Vast plain (6)
I believe the answer is:
steppe
'vast plain' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for steppe that I've seen before include "Large treeless area of Siberia" , "Large area of unforested grassland" , "crosses Siberia" , "Dry grassy plain of central Europe" , "Large area of flat grassland in Europe or Siberia" .)