My Hamster Has A Cut On Its Back What Is It And What Should I Do ?

My Hamster is over a year of age he is always sleeping now, I got him out in his ball for a short while and noticed he has a big cut on his back.I was getting worried because I have hamster before but it sadly died with a disease called wet tail but I don't know what it is and what is and what I should do about this big cut If anyone nos please let me know
Thank you !