Boy happy but not good (3)
I believe the answer is:
lad
'boy' is the definition.
(informal term for boy)
'happy but not good' is the wordplay.
'happy' becomes 'glad' (similar in meaning).
'but not' suggests deleting specific letters.
'good' becomes 'g' (abbreviation).
'glad' with 'g' taken away is 'LAD'.
(Other definitions for lad that I've seen before include "Chap, fellow" , "Boy, young man" , "Stable worker" , "Scottish lover" , "Boy, familiar" .)