An approach to opera (8)
I believe the answer is:
overture
'an approach' is the definition.
(I know that approach can be written as overture)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for overture that I've seen before include "Approach made with view to starting relationship" , "used in attempt to seduce?" , "Approach - musical introduction" , "Instrumental prelude" , "Musical prelude to opera or ballet" .)