Happen once again (5)
I believe the answer is:
recur
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">happen once again</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 recur that I've seen before include "Come round again" , "come back to one's mind" , "Repeat" , "expected to return" , "Return to mind" .)