Flare up (5)
I believe the answer is:
erupt
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">flare up</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for erupt that I've seen before include "be active" , "Burst forth violently" , "Burst forth, like 25" , "Break out like Etna" , "go up" .)