Flat hill (4)
I believe the answer is:
mesa
'flat hill' 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 "Isolated flat-topped hill" , "In the USA, a steep-sided flat-topped hill" , "Flat tableland with steep edges" , "Flat-topped mountain" , "High land" .)