Possible Clues:

• Singer whose name becomes a city if you add an 'R' in the middle
• Eminem guested on his song “Smack That”
• "Don't Matter" rapper
• "Locked Up" rapper
• One-named singer with the 2007 #1 hit 'Don't Matter'
• 'Smack That' singer
• He was on Gwen Stefani's "The Sweet Escape"
• R&B singer with the five-album project 'Stadium'
• Rapper with the debut album "Trouble"
• "Don't Matter" singer/rapper