Sweet sound of mythical bird (4)
I believe the answer is:
rock
'sweet' is the definition.
(as in a stick of rock)
'sound of mythical bird' is the wordplay.
'sound of' shows a homophone (sound like).
'mythical bird' becomes 'roc' (mythical giant bird).
'roc' sounds like 'ROCK'.
(Other definitions for rock that I've seen before include "Seaside confectionery" , "musical genre?" , "Brighton - (novel); - Hudson" , "Gently move; boulder" , "Be unstable; lull (baby)" .)