Intensify, increase rapidly (8)
I believe the answer is:
escalate
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">intensify</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that intensify can be written as escalate)</span><br/ ><br/ >'<span class="explanation-format__original">increase rapidly</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for escalate that I've seen before include "Quickly increase in scale" , "Increase in intensity or extent, like trouble brewing" , "Amplify" , "mount in tension" , "Magnify" .)