Bill that is originally unpaid for so long (5)
I believe the answer is:
adieu
'so long' is the definition.
('adieu' can be a synonym of 'so long')
'bill that is originally unpaid' is the wordplay.
'bill' becomes 'ad' (advertisement stuck on wall).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'originally' suggests taking the first letters.
The initial letter of 'unpaid' is 'u'.
'ad'+'ie'+'u'='ADIEU'
'for' acts as a link.
(Other definitions for adieu that I've seen before include "Goodbye, monsieur" , "Farewell to France" , "Farewel" , "A valediction" , "Parting greeting" .)