Introductory musical piece (7)
I believe the answer is:
prelude
'introductory musical piece' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for prelude that I've seen before include "Opening piece" , "Introductory piece of music" , "Introductory musical piece" , "Curtain-raiser" , "Introductory passage" .)