If a cat loses a whole patch of fur from its ear canal, this could be caused by a fungal infection or by an external injury.
1. First, check the area around the hair loss for any wounds or obvious scabs. If the cat has not been injured recently, then disinfect the wound and put on a protective cover to prevent the cat from scratching and causing further infection. 
2. If it is a fungal skin disease, first clean the hair around the area where hair has fallen out, then apply antifungal medication, and use vitamin B to help the cat restore hair growth. The cat's environment should be disinfected frequently or regularly.
3. Another possibility is that the cat's hair loss is caused by stress and anxiety. In this case, pay attention to whether there have been any changes in the cat's mood recently. You can also supplement the cat's diet with more nutrients to see if it is lacking any substances.
