Monument Valley feature (4)
I believe the answer is:
mesa
'monument valley feature' 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 "In the USA, a steep-sided flat-topped hill" , "In the US south-west, a steep-sided flat-topped hill" , "High land" , "Geographical feature" , "Isolated flat-topped hill" .)