Musical introduction (8)
'musical introduction' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for overture that I've seen before include "Opening gambit" , "Orchestral composition as prelude, or a friendly advance" , "Opera opening" , "used in attempt to seduce?" , "Proposal" .)
(Other definitions for prelude that I've seen before include "Introductory musical piece" , "Introductory passage" , "(Musical) introduction" , "Introductory part, say of poem" , "Introductory piece of music" .)