Possible Clues:

• Problem when using a well?
• Stain from a 12-Down
• What a pocket protector may protect against
• Plant blemish
• Stain that's hard to remove
• Small stain or blot
• Laundry challenge
• Tattoos big belly making dark marking
• Pen user's stain
• Mark 'Waterloo Sunset' group losing head taking drug