### Clue: Selfie regularly appears in mother's quarter (6)
### Answer: MUSLIM 
 ### Explanation: Definition: quarter
An envelope ('appears in') of SLI ('**S**e**L**f**I**e regularly') in MUM ('mother'). About a 'quarter' of the world's population is MUSLIM, but the definition here refers to the thematic 'quarter' of the Old City of Jerusalem.