Good boy is pleased (4)
I believe the answer is:
glad
'is pleased' is the definition.
(similar in meaning)
'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 "Feeling pleasure" , "part of Iris's family" , "Little flowering plant" , "These rags are dressy clothes - happy?" , "Feeling happy" .)