Wagnerian heroine that is about to be disposed of (6)
I believe the answer is:
isolde
'wagnerian heroine' is the definition.
(I've seen this in another clue)
'that is about to be disposed of' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'about' indicates putting letters inside.
'to be disposed of' becomes 'sold' (I've seen this in another clue).
'ie' enclosing 'sold' is 'ISOLDE'.
(Other definitions for isolde that I've seen before include "Soiled like Tristram's girl friend" , "Tristan and ... (Wagner opera)" , "Tristan's beloved" , "Tristan's lover, sadly soiled" , "princess of legend" .)