Charm of baroque tuba: euphony in extremis ! (6)
I believe the answer is:
beauty
'charm' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
(Other definitions for beauty that I've seen before include "See 12" , "Opposite fairy tale characters" , "sometimes put before queen?" , "Supremely pleasing quality" , "- Salon; - spot" .)