Note opening result (5)
I believe the answer is:
event
'result' is the definition.
(I know that result can be written as event)
'note opening' is the wordplay.
'note' becomes 'e' (musical note).
'opening' becomes 'vent' (vent is a kind of opening).
'e'+'vent'='EVENT'
(Other definitions for event that I've seen before include "Incident, occurrence" , "Something happening" , "Item in sports programme" , "Happening, noteworthy perhaps" , "High jump, say" .)