A girl who takes the right part (6)
I believe the answer is:
esther
'a girl' is the definition.
'takes the right part' is the wordplay.
'part' says the answer is hidden in the clue.
'ESTHER' can be found hidden inside 'takes the right'.
'who' is the link.
(Other definitions for esther that I've seen before include "OT heroine" , "10 Heroine" , "Oratorio by Handel; girl's name" , "Beautiful lady" , "Old Testament book that tells of a beautiful Jewess" .)