Bill's name instead of Rex turning up (6)
I believe the answer is:
advent
'rex turning up' is the definition.
I can't judge whether this definition defines the answer.
'bill's name instead' is the wordplay.
'bill' becomes 'ad' (advertisement stuck on wall).
'name instead' becomes 'vent' (I can't justify this - if you can you should believe this answer much more).
'ad'+'vent'='ADVENT'
'of' is the link.
Can you help me to learn more?
(Other definitions for advent that I've seen before include "Pre-Christmas season for Christians" , "Period in the Christian calendar" , "Christian season" , "Pre-Christmas period" , "Season preceding Christmas" .)