Happy? Good boy! (4)
I believe the answer is:
glad
'happy?' is the definition.
(I know that happy can be written as glad)
'good boy' is the wordplay.
'good' becomes 'g' (abbreviation).
'boy' becomes 'lad' (informal term for boy).
'g'+'lad'='GLAD'
(Other definitions for glad that I've seen before include "Informal flower < -girl" , "These rags are dressy clothes - happy?" , "Made up" , "Elated" , "Pleased, delighted" .)