Possible Clues:

• Solar rays
• Benefit of a southern exposure
• Source of vitamin D
• Dracula's worry
• What floods into a glass house
• Paper set on fire is source of warmth, naturally
• Sounds as if your boy, although bright, carries little weight
• Hustling needed to get heater
• Source of 31-Down
• One in France getting in very small amount of illumination during the day