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' is an anagram indicator.
'by' says to put letters next to each other.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'rheingold' with its final letter taken off is 'rheingol'.
'rheingol' with letters rearranged gives 'lohengri'.
'lohengri'+'n'='LOHENGRIN'
(Other definitions for lohengrin that I've seen before include "Musical entertainment" , "Wagnerian opera" , "Wagner opera" .)