Good boy, happy (4)
I believe the answer is:
glad
'happy' is the definition.
('glad' can be a synonym of 'happy')
'good boy' is the wordplay.
'good' becomes 'g' (abbreviation).
'boy' becomes 'lad' (lad is a kind of boy**).
'g'+'lad'='GLAD'
(Other definitions for glad that I've seen before include "Dressy clothes, informally .... rags" , "Feeling happy" , "Informal flower < -girl" , "part of Iris's family" , "These rags are dressy clothes - happy?" .)