Serious, sincere (7)
I believe the answer is:
earnest
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">serious</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >'<span class="explanation-format__original">sincere</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that sincere can be written as earnest)</span><br/ ><br/ ></span>
(Other definitions for earnest that I've seen before include "Sincere; enthusiastic" , "Intensely serious; token" , "Art worthy" , "money involved in contract" , "Showing sincere and intense conviction" .)