First good boy is happy (4)
I believe the answer is:
glad
'is happy' is the definition.
(synonyms)
'first good boy' is the wordplay.
'first good' becomes 'g' (lst letter of 'good').
'boy' becomes 'lad' (informal term for boy).
'g'+'lad'='GLAD'
(Other definitions for glad that I've seen before include "Little flowering plant" , "Informal flower < -girl" , "Dressy clothes, informally .... rags" , "Elated" , "Happy, pleased" .)