Commercial song (6)
I believe the answer is:
jingle
'commercial song' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for jingle that I've seen before include "Catchy verse in advert" , "Catchy slogan or tune" , "Easily-remembered slogan" , "Metallic sound as of coins in pocket" , "upset Pickwick" .)