Yseult's other name is Old English (6)
I believe the answer is:
isolde
'yseult's other name' is the definition.
'is old english' is the wordplay.
'english' becomes 'e' (abbreviation).
'is'+'old'+'e'='ISOLDE'
(Other definitions for isolde that I've seen before include "Tristan's lover, sadly soiled" , "princess of legend" , "Soiled like Tristram's love" , "Tristan and . . . . . . (medieval romance)" , "heroine (sic)" .)