Appearance of commercial opening (6)
I believe the answer is:
advent
'appearance' is the definition.
(I know this)
'commercial opening' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'opening' becomes 'vent' (vent is a kind of opening).
'ad'+'vent'='ADVENT'
'of' acts as a link.
(Other definitions for advent that I've seen before include "when the Church year begins" , "Forty days preceding Christmas" , "Pre-Christmas period" , "Run-up to Christmas" , "Period in the Christian calendar" .)