Maybe you could post a picture so that we could see what you mean? I have had a hamster with an recurring abscess before. I treated it by putting coconut oil on it every day for around a week or so. This helped a lot. Though from the area you said it is this might have something to do with his/her scent glands. I scanned this website and it appears to have some helpful info on the subject:
What You May Not Know About Hamster Scent Glands - Veterinary Practice News - March 2012
I am no expert though, I hope you find what you are looking for!