Calm or tranquil (6)
I believe the answer is:
placid
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">calm</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 placid)</span><br/ ><br/ ></span>
(Other definitions for placid that I've seen before include "Collected" , "Peaceful and tranquil" , "Not easily upset or excited" , "Calm; equable" , "Tranquil, even-tempered" .)