Popular girl breaking into number in which all join (9)
I believe the answer is:
sing-along
'which all join' is the definition.
I know nothing about this answer so I can't judge whether this works.
'popular girl breaking into number' is the wordplay.
'popular' becomes 'in'.
'girl breaking' becomes 'gal' (I can't justify this - if you can you should believe this answer much more).
'into' means one lot of letters goes inside another.
'number' becomes 'song' (I've seen this before).
'in'+'gal'='ingal'
'ingal' inserted into 'song' is 'SING-ALONG'.
'in' is the link.
(Other definitions for sing-along that I've seen before include "Informal group musical activity" , "Form of audience participation" , "community music event?" , "Join in with performer" .)