Prelude to an opera (8)
I believe the answer is:
overture
'prelude to an opera' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for overture that I've seen before include "Orchestral introduction to an opera" , "Introduction to, eg, an opera" , "Instrumental prelude" , "10 {COMPOSITION]" , "offer" .)