The sound of music? (4)
This clue is a double definition.
'the sound' is the first definition.
(pop is a kind of sound)
'music' is the second definition.
(pop music)
'of' acts as a link.
(Other definitions for pop that I've seen before include "Music with mass appeal" , "Old jingle - . . . goes the weasel" , "Palindrome of music for the young" , "Explode; Eton society" , "Explosive sound; drink" .)
(Other definitions for tune that I've seen before include "Optimise the condition of" , "Air, melody" , "Song" , "Melody; adjust (engine)" , "temper" .)