Flat formation (4)
'flat formation' 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 "Isolated flat-topped hill" , "Flat tableland with steep sides" , "In the US south-west, a steep-sided flat-topped hill" , "Flat-topped mountain" , "Flat hill" .)
(Other definitions for plateau that I've seen before include "Quiet time" , "Levelling off" , "High plain" , "Reach a level" , "Elevated flat land" .)