Key opening occasion (5)
I believe the answer is:
event
'occasion' is the definition.
(I've seen this before)
'key opening' is the wordplay.
'key' becomes 'e' (musical key).
'opening' becomes 'vent' (vent is a kind of opening).
'e'+'vent'='EVENT'
(Other definitions for event that I've seen before include "Affair" , "tournament" , "Occurence, occasion" , "case" , "important" .)