Possible Clues:

• 'One's alternative personality (5,3)'
• What psychoanalysis might do for sign of split personality?
• Bruce Wayne, for one
• Very close friend who seems almost part of oneself
• 'One's second self (5,3)'
• Modify, say, second character in novel, such as Mr. Hyde
• One's second personality
• Intimate confidante
• A toy has to soften up Achates
• 19-Down, for one