An opening celebration or its finale (4)
I believe the answer is:
door
'an opening' is the definition.
(a door is an opening)
'celebration or its finale' is the wordplay.
I cannot really understand how this works, but
'celebration' could be 'do' (I've seen this in other clues) and 'do' is found in the answer.
'or' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for door that I've seen before include "Alternative to knocker" , "Portal" , "Jim Morrison?" , "See 1" , "Hinged barrier" .)