Song of happiness (4)
I believe the answer is:
glee
This clue is a double definition.
'song' is the first definition.
(I know that glee is a type of partsong)
'happiness' is the second definition.
(I know that glee is a type of gaiety)
'of' acts as a link.
(Other definitions for glee that I've seen before include "Unholy delight" , "Mirth - part-song" , "Old English part-song" , "Lively or triumphant joy" , "Great merriment" .)