Commercial outlet in run-up to Christmas (6)
I believe the answer is:
advent
'run-up to christmas' is the definition.
(I've seen this before)
'commercial outlet' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'outlet' becomes 'vent' (I've seen this before).
'ad'+'vent'='ADVENT'
'in' is the link.
(Other definitions for advent that I've seen before include "when the Christmas shopping has to be done?" , "observed by Christian church" , "Arrival of important person or event" , "Church season" , "sort of calendar" .)