Onset of publicity prior to opening (6)
I believe the answer is:
advent
'onset' is the definition.
(I've seen this in another clue)
'publicity prior to opening' is the wordplay.
'publicity' becomes 'ad' (short for advert).
'prior to' means one lot of letters go next to another.
'opening' becomes 'vent' (a vent is an opening or outlet).
'ad'+'vent'='ADVENT'
'of' is the link.
(Other definitions for advent that I've seen before include "Christmas shopping time in" , "Coming - occasion in Church calendar" , "Forty days preceding Christmas" , "Church season" , "The season leading up to Christmas" .)