Close, airless (6)
I believe the answer is:
stuffy
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">close</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(both can mean uncomfortably warm)</span><br/ ><br/ >'<span class="explanation-format__original">airless</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for stuffy that I've seen before include "Lacking fresh air or excessively conventional" , "Strait-laced -- lacking fresh air" , "Airless - humourless" , "Close, airless" , "Close - conventional and dull" .)