Chance happening (5)
I believe the answer is:
fluke
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">chance happening</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 fluke that I've seen before include "Parasitic flatworm" , "parasitic worm!" , "Lucky stroke - part of anchor" , "land, by chance" , "Accidental stroke of good luck, or part of whale's tail" .)