Southwestern plateau (4)
I believe the answer is:
mesa
'southwestern plateau' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mesa that I've seen before include "Flat-topped hill with steep sides, common in the southwestern US" , "Flat-topped mountain" , "High land" , "Flat tableland with steep sides" , "Flat hill" .)