Number boy before girl, initially (4)
I believe the answer is:
song
'number' is the definition.
(thesaurus)
'boy before girl initially' is the wordplay.
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'before' is a charade indicator (letters next to each other).
'initially' suggests taking the first letters.
The first letter of 'girl' is 'g'.
'son'+'g'='SONG'
(Other definitions for song that I've seen before include "Ditty" , "Ballad, aria" , "Piece of music, usually with words" , "Words set to music" , "Carol, for instance" .)