Child with good number (4)
I believe the answer is:
song
'number' is the definition.
(musical number)
'child with good' is the wordplay.
'child' becomes 'son' (male child).
'with' means one lot of letters go next to another.
'good' becomes 'g' (abbreviation).
'son'+'g'='SONG'
(Other definitions for song that I've seen before include "Tune" , "You work best when on this" , "Something recorded by 26 [FACES] , for example" , "Piece of vocal music" , "Vocal composition" .)