They happen to be key openings (6)
I believe the answer is:
events
'they happen' is the definition.
(I've seen this before)
'key openings' is the wordplay.
'key' becomes 'e' (musical key).
'openings' becomes 'vents' (vent is a kind of opening).
'e'+'vents'='EVENTS'
'to be' is the link.
(Other definitions for events that I've seen before include "Things taking place" , "competition" , "Horse-riding competitions" , "Things that happen" , "races?" .)