It doesn't look like an open wound, just a patch of inflamed skin without fur, so not likely to become infected unless she keeps scratching it to a point where the skin splits and becomes an open wound.
I would say from experience that it is most likely a case of mites. Hamsters are prone to mites which cause them to scratch excessively and thus loose fur. I would suggest trying a topical spot on treatment such as 'Xeno 50 Mini Spot On' which you can buy online from Pets At Home, or on Amazon (Amazon is slightly cheaper then Pets At Home):
https://www.amazon.co.uk/gp/product/...?ie=UTF8&psc=1
I would recommend trying this as a first course of action, especially as you mentioned that you have seen your hamster scratching the area repeatedly. In my opinion it is most likely mites, but the good thing is that it can be treated easily with the above mentioned treatment (this is what a vet would use). There are cheaper versions available, such as Beaphar Anti Parasite Spot On, but this has a lower amount of Ivermectin in it and is not as good for treatment as it is more for preventative:
https://www.viovet.co.uk/Beaphar-Ant...caArV1EALw_wcB
But also just to be sure, check the inside of her cheek pouch to make sure that there isn't anything sharp or otherwise lodged inside it, which could be the cause of her excessive scratching of that area. You can check her cheek pouch by gently feeling the area with your fingertips.