Calm, tranquil (6)
I believe the answer is:
placid
<span class="explanation-format__ClueExplanation">I believe 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">(synonyms)</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 "mild" , "Calm and not easily upset" , "Quiet and still" , "Not ruffled" , "Calm, peaceful" .)