Sharp or severe (5)
I believe the answer is:
acute
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">sharp</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >'<span class="explanation-format__original">severe</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for acute that I've seen before include "Sharp-pointed - keenly perceptive" , "Incisive" , "Sharp, angle or mind" , "quick on the uptake" , "Sharp; type of accent" .)