Calm setting for commercial song (8)
I believe the answer is:
serenade
'song' is the definition.
(I know that serenade is a type of song)
'calm setting for commercial' is the wordplay.
'calm' becomes 'serene' (synonyms).
'setting for' means one lot of letters goes inside another.
'commercial' becomes 'ad' (abbreviation for advertisement).
'serene' going around 'ad' is 'SERENADE'.
(Other definitions for serenade that I've seen before include "Evening love song" , "Evening music (played outside woman's house)" , "Song of romance" , "Music appropriate for evening" , "(Romantic?) song or tune" .)