Early rock music? (6-4)

I believe the answer is:

cradle song

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'early rock music?' is the definition.
'cradle song' can be an answer for 'music?' (cradle song is a kind of music). I am not certain of the 'early rock' bit.

'early rock music?' is the wordplay.
'early rock' becomes 'cradle' (synonyms. I am not sure about the 'early' bit.).
'music?' becomes 'song' (song is a kind of music).
'cradle'+'song'='CRADLE-SONG'

Can you help me to learn more?

(Another definition for cradle song that I've seen is " Lullaby".)

I've seen this clue in The Telegraph.

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