Simple flat area (5)
I believe the answer is:
plain
I believe this clue is a double definition.
'simple' is the first definition.
(synonyms)
'flat area' is the second definition.
(I've seen this before)
(Other definitions for plain that I've seen before include "Unvarnished" , "no oil painting features" , "not striking" , "Ordinary; area of land" , "Basic" .)