Happening to have a key and opening (5)
I believe the answer is:
event
'happening' is the definition.
(I've seen this before)
'a key and opening' is the wordplay.
'a key' becomes 'E' (a musical key).
'and' says to put letters next to each other.
'opening' becomes 'vent' (vent is a kind of opening).
'e'+'vent'='EVENT'
'to have' acts as a link.
(Other definitions for event that I've seen before include "Significant happening" , "ride competitively" , "special occasion" , "What will happen" , "A thing taking place" .)