It happens to be a key opening (5)
I believe the answer is:
event
'it happens' is the definition.
(I've seen this before)
'a key opening' is the wordplay.
'a key' becomes 'e' (musical key).
'opening' becomes 'vent' (vent is a kind of opening).
'e'+'vent'='EVENT'
'to be' acts as a link.
(Other definitions for event that I've seen before include "horse-riding competition" , "Planned public or social occasion" , "'Occurrence, occasion (5)'" , "Particular happening" , "pole vault maybe" .)