Happening without warning (6)
I believe the answer is:
sudden
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">happening without warning</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for sudden that I've seen before include "'Unexpectedly, without warning (6)'" , "Happening quickly and without warning" , "Occurring quickly without warning" , "Abrupt, unexpected" , "Happening abruptly" .)