Rock music of the quieter type (7)

I believe the answer is:

lullaby

I'm a little stuck... Click here to teach me more about this clue!

'rock music' is the definition.
'lullaby' can be an answer for 'music' (lullaby is a kind of music). I am unsure of the 'rock' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for lullaby that I've seen before include "Gentle song to put baby to sleep" , "Cradle-song" , "Soothing song for child" , "Song for pacifying baby" , "Berceuse" .)

I've seen this clue in The Globe and Mail.

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