System converting song into aria (12)
I believe the answer is:
organisation
'system' is the definition.
(I know that system can be written as organisation)
'converting song into aria' is the wordplay.
'converting' is an anagram indicator.
'song'+'into'+'aria'='songintoaria'
'songintoaria' anagrammed gives 'ORGANISATION'.
(Other definitions for organisation that I've seen before include "System" , "Running" , "concern" , "Systematic approach to tasks" .)