Flat-topped hill (4)
I believe the answer is:
mesa
'flat-topped hill' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mesa that I've seen before include "Flat-topped mountain" , "Flat tableland with steep edges" , "US steep-sided tableland" , "High land" , "In the US south-west, a steep-sided flat-topped hill" .)