Period before Christmas opening, after a dubious beginning (6)
I believe the answer is:
advent
'period before christmas' is the definition.
(I've seen this before)
'opening after a dubious beginning' is the wordplay.
'opening' becomes 'vent' (vent is a kind of opening**).
'after' is a charade indicator (letters next to each other).
'beginning' suggests taking the first letters.
The first letter of 'dubious' is 'd'.
'a'+'d'='ad'
'vent' put after 'ad' is 'ADVENT'.
(Other definitions for advent that I've seen before include "appearance" , "Forty days preceding Christmas" , "Run-up to Christmas" , "approach" , "Pre-Christmas period" .)