It happens (5)
'it happens' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for event that I've seen before include "celebration?" , "contest in programme" , "Happening, noteworthy perhaps" , "game, perhaps" , "A thing taking place" .)
(Other definitions for fit that I've seen before include "Certified OK by the doctor" , "Of suitable quality" , "In good physical shape 93)" , "Healthy; spasm" , "Apt" .)