Tristan’s lover, one deceived by English (6)
I believe the answer is:
isolde
'tristan's lover' is the definition.
(I've seen this before)
'one deceived by english' is the wordplay.
'one' becomes 'i' (Roman numeral).
'deceived' becomes 'sold' (selling is a kind of deceiving).
'by' says to put letters next to each other.
'english' becomes 'e' (abbreviation).
'i'+'sold'+'e'='ISOLDE'
(Other definitions for isolde that I've seen before include "princess of legend" , "Soiled like Tristan's lover" , "Tristan's beloved" , "for Tristram" , "Soiled like Tristram's girl friend" .)