Love song composed about Bill (8)
I believe the answer is:
serenade
'love song' is the definition.
(I've seen this before)
'composed about bill' is the wordplay.
'composed' becomes 'serene' (I've seen this before).
'about' is an insertion indicator.
'bill' becomes 'ad' (advertisement stuck on wall).
'serene' going around 'ad' is 'SERENADE'.
(Other definitions for serenade that I've seen before include "It's played or sung at a lover's window" , "Sing (for someone on a balcony?)" , "Romantic song under fair lady's window" , "composition" , "Evening music (played outside woman's house)" .)