It happens to have a key-hole (5)
'it happens' is the definition.
(I've seen this before)
'a key-hole' is the wordplay.
'a key' becomes 'e' (I've seen this in other clues).
'hole' becomes 'vent' (vent is a kind of hole).
'to have' acts as a link.
(Other definitions for event that I've seen before include "400 metres?", "Significant happening", "case", "Incident - final outcome", "celebration?".)