Performer completely at sea about Rossini’s finale (7)
I believe the answer is:
soloist
'performer' is the definition.
(I have seen 'Lone performer' mean 'soloist' so perhaps 'performer' could also mean 'soloist')
'completely at sea about rossini's finale' is the wordplay.
'completely' becomes 'so' (both can mean 'very much').
'at sea' becomes 'lost' (I've seen this before).
'about' is an insertion indicator.
'finale' suggests the final letters.
The final letter of 'rossini' is 'i'.
'lost' enclosing 'i' is 'loist'.
'so'+'loist'='SOLOIST'
(Other definitions for soloist that I've seen before include "this one's on his own" , "Virtuoso player" , "Single performer" , "Lone performer" , "Only one playing" .)