Opening note penned by German composer (6)
I believe the answer is:
breach
'opening' is the definition.
(breach is a kind of opening)
'note penned by german composer' is the wordplay.
'note' becomes 're'.
'penned by' indicates putting letters inside.
'german composer' becomes 'bach' (I've seen this before).
're' inserted within 'bach' is 'BREACH'.
(Other definitions for breach that I've seen before include "The breaking of a law or agreement" , "Violation, maybe of the peace" , "Violation of a promise" , "'Gap, break or hole (6)'" , "Break (in a wall, etc)" .)