Only a small area, and that's flat! (5)
I believe the answer is:
facet
'flat' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for facet that I've seen before include "Particular aspect" , "Small flat surface of cut gemstone" , "Side or surface, diamond or building" , "Distinct feature" , "Smooth surface of a gem" .)