Although flannel is properly a textile, "a flannel" is a shirt made from flannel, with a tartan pattern. They're fairly warm shirts, and often associated with farming or even bogans (we call the shirts "flannies" in Australia). "Clad" means "clothed", but the word clad also implies a number of protective covers: "clad in armour". So "flannel-clad" just means the person is wearing a flannel shirt. This can imply something about the person's character (ie. the type of person who would wear a flannel shirt), but it's hard to tell without context. When I was last in Australia, it was suddenly very fashionable for teenage girls to wear flannies.