Obvious upset originally about opening piece (8)
I believe the answer is:
overture
'piece' is the definition.
(I've seen this before)
'obvious upset originally about opening' is the wordplay.
'obvious' becomes 'overt' (similar in meaning).
'upset originally about opening' becomes 'ure' (I can't justify this - if you can you should believe this answer much more).
'overt'+'ure'='OVERTURE'
Can you help me to learn more?
(Other definitions for overture that I've seen before include "Music played before an opera" , "Musical prelude to opera or ballet" , "used in attempt to seduce?" , "Orchestral work" , "Item of music" .)