A key opening for period before Christmas (6)
I believe the answer is:
advent
'period before christmas' is the definition.
(I've seen this before)
'a key opening' is the wordplay.
'key' becomes 'd' (I've seen this in other clues).
'opening' becomes 'vent' (vent is a kind of opening**).
'a'+'d'+'vent'='ADVENT'
'for' acts as a link.
(Other definitions for advent that I've seen before include "Church season" , "sort of calendar" , "Arrival of important person or event" , "The coming of a religous period?" , "Christian season" .)