Peaceful, tranquil (6)
I believe the answer is:
serene
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">peaceful</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >'<span class="explanation-format__original">tranquil</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that tranquil can be written as serene)</span><br/ ><br/ ></span>
(Other definitions for serene that I've seen before include "Calm and unclouded" , "Not agitated" , "Totally unagitated" , "Peaceful and contented" , "still" .)