Extreme; serious (7)
I believe the answer is:
intense
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">extreme</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >'<span class="explanation-format__original">serious</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for intense that I've seen before include "Forceful, concentrated" , "Profound" , "Great" , "In an extreme degree" , "Acute, concentrated" .)