Not pretty flat? (5)
I believe the answer is:
plain
'flat?' is the definition.
(I know that un-notched can be written as plain)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for plain that I've seen before include "Clear, candid" , "Unadorned on flat area" , "Undecorated; flat area" , "Simple, unadorned" , "Unvarnished" .)