Serene, unruffled (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">serene</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">unruffled</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for placid that I've seen before include "Calm, unruffled" , "Cool and unruffled" , "Calm, peaceful" , "Calm and not easily upset" , "Quiet and still" .)