Just Barely There by MagnificentAndStrange
Summary: Harry’s life at the Dursleys is bad enough that his magic is so used to keeping him alive that when he gets to Hogwarts his magic is noticeably weaker than other students, or completely ineffective. Harry is considered a disappointment and a freak by many people who had thought he would be a hero and powerful because he ‘vanquished’ Voldemort. Snape never had any illusions about Harry being a hero and thus is in a unique position to realize that the boy’s magic is being almost completely spent on protecting Harry from harm. As Snape confronts his own issues regarding Harry Potter, will he be able to help the first year heal from trauma? Will Harry’s magic ever return to normal?