Lover I betrayed — conclusion of romance (6)
I believe the answer is:
isolde
'lover' is the definition.
(one of the lovers in the opera Tristan and Isolde)
'i betrayed conclusion of romance' is the wordplay.
'betrayed' becomes 'sold' ('sell' can be a synonym of 'betray').
'conclusion of' suggests the final letters.
The final letter of 'romance' is 'e'.
'i'+'sold'+'e'='ISOLDE'
(Other definitions for isolde that I've seen before include "love potion drinker" , "Tristan and . . . . . . (Wager opera)" , "Tristan and ... (Wagner opera)" , "Soiled like Tristram's girl friend" , "Arthurian queen" .)