Sincerely, seriously (9)
I believe the answer is:
earnestly
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">sincerely</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">seriously</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that seriously can be written as earnestly)</span><br/ ><br/ ></span>
(Other definitions for earnestly that I've seen before include "Showing serious intent" , "with heart and soul" , "Showing commitment" , "Sincerely and seriously" , "how Wilde character may be acted?" .)