Happily apt to burst into song (7)
I believe the answer is:
gleeful
'happily apt to burst' is the definition.
I can't tell whether this definition defines the answer.
'song' is the wordplay.
I cannot quite understand how this works, but
'song' could be 'glee' (I've seen this before) and 'glee' is found within the answer.
This may be the basis of clue (or it may be nonsense).
'into' is the link.
Can you help me to learn more?
(Other definitions for gleeful that I've seen before include "Jubilant" , "Exuberantly joyous" , "Exuberantly joyful" , "Happy" , "Cheerful" .)