German composer ringing about opening (6)
I believe the answer is:
breach
'opening' is the definition.
(breach is a kind of opening)
'german composer ringing about' is the wordplay.
'german composer' becomes 'bach' (I've seen this before).
'ringing' indicates putting letters inside.
'about' becomes 're' ('re' means regarding or about).
'bach' going around 're' is 'BREACH'.
(Other definitions for breach that I've seen before include "Gap (in defensive wall)" , "Infringement - gap" , "'Gap, break or hole (6)'" , "The breaking of a law or agreement" , "Non-observance" .)