The girl in the song (5)
I believe the answer is:
carol
I believe this is a double definition.
'the girl' is the first definition.
(woman's name)
'the song' is the second definition.
(carol is a kind of song)
'in' acts as a link.
(Other definitions for carol that I've seen before include "Canticle" , "Joyful hymn" , "Female" , "Girl's name; sing" , "Song celebrating the birth of Jesus" .)