Good boy is happy (4)
I believe the answer is:
glad
'happy' is the definition.
(similar in meaning)
'good boy' is the wordplay.
'good' becomes 'g' (abbreviation).
'boy' becomes 'lad' (lad is a kind of boy).
'g'+'lad'='GLAD'
'is' acts as a link.
(Other definitions for glad that I've seen before include "More than willing" , "Made up" , "Feeling pleasure" , "Pleased, delighted" , "part of Iris's family" .)