Full of happiness (6)
'full of happiness' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for joyous that I've seen before include "Cock-a-hoop" , "Happy; festive" , "Full of happiness" , "Exultant" , "Enraptured" .)
(Other definitions for joyful that I've seen before include "Feeling great happiness" , "Happy" , "Elated" , "Feeling rapture or delight" , "Full of glee" .)