Happy to be a good boy (4)
I believe the answer is:
glad
'happy' is the definition.
(synonyms)
'a good boy' is the wordplay.
'a good' becomes 'g' (abbreviation).
'boy' becomes 'lad' (lad is a kind of boy).
'g'+'lad'='GLAD'
'to be' is the link.
(Other definitions for glad that I've seen before include "Dressy clothes, informally .... rags" , "Happy, pleased" , "Little flowering plant" , "Made up" , "part of Iris's family" .)