Extra note is switched for two (8)
I believe the answer is:
residual
'extra' is the definition.
(I know that extra can be written as residual)
'note is switched for two' is the wordplay.
'note' becomes 're' (as in do-re-mi).
'switched' says the letters should be written backwards.
'for two' becomes 'dual' (I've seen this in another clue).
'is' back-to-front is 'si'.
're'+'si'+'dual'='RESIDUAL'
(Other definitions for residual that I've seen before include "Pertaining to what is left over" , "Concerning what is left over" , "Left over as a remainder" , "'Remaining, left over (8)'" , "Remaining; repeat royalty" .)