Composer badly taken aback after opening of music omitted (7)
I believe the answer is:
corelli
'composer' is the definition.
(Arcangelo Corelli)
I don't understand the remainder of the clue.
(Other definitions for corelli that I've seen before include "Arcangelo --, Italian baroque composer" , "Arcangelo, seventeenth to eighteenth century Italian composer, violinist" , "A mandolin-playing Captain" , "Marie - - , romantic 17" , "Captain -'s Mandolin (de Bernires)" .)