Chance happening (5)
I believe the answer is:
fluke
'chance happening' is the definition.
(I've seen this in another clue)
This is all the clue.
(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" .)