Serious, extreme (6)
I believe the answer is:
severe
<span class="explanation-format__ClueExplanation">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">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">extreme</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for severe that I've seen before include "Austere or strict" , "Strict, hard" , "Extreme - strict" , "Austere, spartan" , "Forbidding in action or appearance" .)