Largest city in the U.S. that's not a county seat (4)
I believe the answer is:
mesa
(Other definitions for mesa that I've seen before include "Flat tableland with steep sides" , "In the USA, a steep-sided flat-topped hill" , "Isolated flat-topped hill" , "Flat-topped hill with steep sides, common in the southwestern US" , "Flat tableland with steep edges" .)