Coming a day before opening (6)
I believe the answer is:
advent
'coming' is the definition.
('advent' can be a synonym of 'coming')
'a day before opening' is the wordplay.
'day' becomes 'd'.
'before' is a charade indicator (letters next to each other).
'opening' becomes 'vent' (vent is a kind of opening**).
'a'+'d'+'vent'='ADVENT'
(Other definitions for advent that I've seen before include "Arrival of important person or event" , "Coming, arrival" , "sort of calendar" , "when the Christmas shopping has to be done?" , "Christian season" .)