My second tune, one from The Pirates of Penzance? (7)
I believe the answer is:
corsair
'one from the pirates of penzance?' is the definition.
('corsair' is an archaic term for a pirate)
'my second tune' is the wordplay.
'my' becomes 'cor' (both are expressions of surprise).
'second' becomes 's' (common abbreviation).
'tune' becomes 'air' (air can mean a piece of music).
'cor'+'s'+'air'='CORSAIR'
(Other definitions for corsair that I've seen before include "work by Verdi" , "Pirate (archaic)" , "Buccaneer" , "A swift pirate ship" , "Main villain" .)