Concerned with part of the orchestra coming back (12)
I believe the answer is:
repercussion
'coming back' is the definition.
(the effects of an event coming back)
'concerned with part of the orchestra' is the wordplay.
'concerned with' becomes 're' (re can mean regarding, concerning).
'part of the orchestra' becomes 'percussion'.
're'+'percussion'='REPERCUSSION'
(Other definitions for repercussion that I've seen before include "Indirect effect; echo" , "Unintended consequence" , "Impact" , "Indirect consequence of some action" , "Backlash" .)