Break out suddenly (5)
I believe the answer is:
erupt
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">break out suddenly</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for erupt that I've seen before include "be active" , "Explode" , "Break out dramatically" , "Burst forth violently" , "Eject lava" .)