The finale sounds like Sibelius, say (6)
I believe the answer is:
finish
'the finale' is the definition.
('finish' can be a synonym of 'finale')
'like sibelius say' is the wordplay.
'like sibelius' becomes 'finnish' (I've seen this in another clue).
'say' indicates a 'sounds like' (homophone) clue.
'finnish' is a homophone of 'FINISH'.
'sounds' is the link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for finish that I've seen before include "End, complete" , "Terminate, conclude" , "End, conclusion" , "Polish, perhaps" , "Cease, conclude" .)