Dances include promotional material for songs (7)
I believe the answer is:
ballads
'songs' is the definition.
(ballad is a kind of song)
'dances include promotional material' is the wordplay.
'dances' becomes 'balls' (ball is a kind of dance).
'include' means one lot of letters goes inside another.
'promotional material' becomes 'ad' (ad is a kind of promotional material).
'balls' going around 'ad' is 'BALLADS'.
'for' acts as a link.
(Other definitions for ballads that I've seen before include "Soppy songs!" , "Popular songs" .)