Bill joins in peaceful song (8)
I believe the answer is:
serenade
'song' is the definition.
(I know that serenade is a type of song)
'bill joins in peaceful' is the wordplay.
'bill' becomes 'ad' (advertisement stuck on wall).
'joins in' indicates putting letters inside.
'peaceful' becomes 'serene' (similar in meaning).
'ad' going inside 'serene' is 'SERENADE'.
(Other definitions for serenade that I've seen before include "Song of romance" , "Evening love song" , "Noted performance" , "Open-air song" , "(Romantic?) song or tune" .)