Tricky to cover bill, unfortunately (5)
I believe the answer is:
sadly
'unfortunately' is the definition.
(I've seen this before)
'tricky to cover bill' is the wordplay.
'tricky' becomes 'sly' (synonyms).
'to cover' is an insertion indicator.
'bill' becomes 'ad' (advertisement stuck on wall).
'sly' enclosing 'ad' is 'SADLY'.
(Other definitions for sadly that I've seen before include "Lamentably, unfortunately" , "Mournfully" , "With sorrow" , "Regrettably" , "I regret to say" .)