Note boy is happy (4)
I believe the answer is:
glad
'happy' is the definition.
(I know that happy can be written as glad)
'note boy' is the wordplay.
'note' becomes 'g' (musical note).
'boy' becomes 'lad' (informal term for boy).
'g'+'lad'='GLAD'
'is' acts as a link.
(Other definitions for glad that I've seen before include "More than willing" , "These rags are dressy clothes - happy?" , "bright?" , "Little flowering plant" , "Informal flower < -girl" .)