Formally correct old part of duet (5)
I believe the answer is:
primo
'part of duet' is the definition.
(I've seen this before)
'formally correct old' is the wordplay.
'formally correct' becomes 'prim' (prim and proper).
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'prim'+'o'='PRIMO'
(Other definitions for primo that I've seen before include "Duet's lead part (It. first)" , "top player" , "Principal part of duet" , "Right-hand part in piano duet" , "... Levi (novelist)" .)