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