Flat-topped hill (4)
I believe the answer is:
mesa
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">flat-topped hill</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)