Breaks out suddenly (6)
I believe the answer is:
erupts
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">breaks 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 erupts that I've seen before include "Breaks out suddenly" , "Blows up like volcano" , "Breaks out abruptly" , "loses temper?" , "Bursts" .)