Rheingold mostly replaced by new Wagner opera (9)
I believe the answer is:
lohengrin
'wagner opera' is the definition.
(I've seen this before)
'rheingold mostly replaced by new' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'replaced' indicates anagramming the letters.
'by' says to put letters next to each other.
'new' becomes 'n' (abbreviation as in New Testament NT).
'rheingold' with its last letter removed is 'rheingol'.
'rheingol' anagrammed gives 'lohengri'.
'lohengri'+'n'='LOHENGRIN'
(Other definitions for lohengrin that I've seen before include "Musical entertainment" , "Wagnerian opera" , "Wagner opera" .)