Steep-sided plateau (4)
I believe the answer is:
mesa
'steep-sided plateau' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mesa that I've seen before include "US steep-sided tableland" , "Isolated flat-topped hill" , "Flat-topped tableland" , "Flat tableland with steep sides" , "In the USA, a steep-sided flat-topped hill" .)