Good boy, eager (4)
I believe the answer is:
glad
'eager' is the definition.
(thesaurus)
'good boy' is the wordplay.
'good' becomes 'g' (abbreviation).
'boy' becomes 'lad' (lad is a kind of boy**).
'g'+'lad'='GLAD'
(Other definitions for glad that I've seen before include "Pleased, delighted" , "Contented" , "Little flowering plant" , "Informal flower < -girl" , "part of Iris's family" .)