Area that’s not; pretty flat? (5)
I believe the answer is:
plain
This might be a double definition.
'area' is the first definition.
(I have seen 'area of land' mean 'plain' so perhaps 'area' could also mean 'plain')
'not pretty flat?' is the second definition.
'plain' can be an answer for 'flat?' (I've seen this in other clues). I'm not sure about the 'not pretty' bit.
'that's' is the link.
Can you help me to learn more?
(Other definitions for plain that I've seen before include "ugly" , "Undecorated; flat area" , "Geographical feature" , "stretch of level land" , "no oil painting features" .)