Your setter replacing one singer with another to make music (9)

I believe the answer is:

bluegrass

I'm a little stuck... Click here to teach me more about this clue!

'music' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for bluegrass that I've seen before include "American hillbilly music" , "Style of US country music" , "A type of country music" , "nickname of US State" , "Kind of country music" .)

I've seen this clue in The Independent.

Want a hint initially instead of a full solution? Install my app