Piano is accompanying contrary characters, overshadowing event (8)
I believe the answer is:
occasion
'event' is the definition.
(occasion is a kind of event)
'piano is accompanying contrary characters overshadowing' is the wordplay.
'overshadowing' indicates a hidden word (I've seen 'overshadows' mean this).
'contrary characters' says the letters should be written in reverse.
'noisacco' can be found hidden inside 'piano is accompanying'.
'noisacco' back-to-front is 'OCCASION'.
(Other definitions for occasion that I've seen before include "Need" , "Time of (maybe special) event" , "Provoke" , "A special event, or the time it happens" , "Social event or occurrence" .)