Simple; undecorated (5)
I believe the answer is:
plain
I believe this clue is a double definition.
'simple' is the first definition.
(similar in meaning)
'undecorated' is the second definition.
(similar in meaning)
(Other definitions for plain that I've seen before include "ugly" , "Flat land; unmarked" , "Ordinary; area of land" , "Ordinary; flat region" , "Clear - open - grassland" .)