Back before beginning of operatic duet part (7)
I believe the answer is:
secondo
'duet part' is the definition.
(I know that secondo is part of duet)
'back before beginning of operatic' is the wordplay.
'back' becomes 'second' (both can mean to support an idea or proposal).
'before' says to put letters next to each other.
'beginning of' says to take the initial letters.
The first letter of 'operatic' is 'o'.
'second'+'o'='SECONDO'
(Other definitions for secondo that I've seen before include "lower part" , "Musical" , "Part played by duettist" , "Part of duet" , "participant in piano duet?" .)