Left on account of mistress? (5)
I believe the answer is:
lover
'mistress?' is the definition.
(thesaurus)
'left on account of' is the wordplay.
'left' becomes 'l' (common abbreviation).
'on account of' becomes 'over'.
'l'+'over'='LOVER'
(Other definitions for lover that I've seen before include "Opposite of hateful type" , "Romeo (or Juliet)" , "Beau" , "Lothario" , "Boyfriend" .)