"The Sound of Music" extra (3)
I believe the answer is:
nun
'the sound of music extra' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nun that I've seen before include "One who's taken vows" , "Carmelite, for example" , "Woman with a habit" , "One in holy order" , "Woman respecting order" .)