Fabulous bird’s music heard (3)
I believe the answer is:
roc
'fabulous bird's' is the definition.
(mythical or fabulous giant bird)
'music heard' is the wordplay.
'music' becomes 'rock' (rock music).
'heard' shows a homophone (sound like).
'rock' sounds like 'ROC'.
(Other definitions for roc that I've seen before include "Legendary air carrier" , "Vast bird of legend" , "Legendary flyer" , "Vast legendary bird" , "Fantastic bird" .)