Boy group's first tune (4)
I believe the answer is:
song
'tune' is the definition.
(thesaurus)
'boy group's first' is the wordplay.
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'first' suggests taking the first letters.
The initial letter of 'group' is 'g'.
'son'+'g'='SONG'
(Other definitions for song that I've seen before include "You work best when on this" , "Carol, for instance" , "Musical composition; air" , "Piece of music, usually with words" , "Musical rendition" .)