Boy group's opening number (4)
I believe the answer is:
song
'number' is the definition.
(thesaurus)
'boy group's opening' is the wordplay.
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'opening' suggests taking the first letters.
The first letter of 'group' is 'g'.
'son'+'g'='SONG'
(Other definitions for song that I've seen before include "Piece of vocal music" , "Ballad, aria" , "Lied" , "Vocal item" , "You work best when on this" .)