Way lad breaks into grand musical sound (8)
I believe the answer is:
birdsong
'musical sound' is the definition.
(birdsong might sound musical)
'way lad breaks into grand' is the wordplay.
'way' becomes 'rd' (short for road).
'lad' becomes 'son' (lad can informally mean a young boy).
'breaks into' indicates putting letters inside (inserted letters break into the word).
'grand' becomes 'big'.
'rd'+'son'='rdson'
'rdson' put within 'big' is 'BIRDSONG'.
(Other definitions for birdsong that I've seen before include "sound of morning, say" , "Twitter" , "tweet" , "Harmonious twittering" , "sound from the skies?" .)