If something's wanting, there's not enough of something necessary in it. If your essay is wanting, it's missing something important and won't get an A. Try adding body paragraphs.
If you forget the sugar in your chocolate chip cookies, you'll find them wanting when you taste them. Likewise, if your favorite team's defense is wanting, they're likely to lose the basketball game. Wanting, which was originally wantand, comes from the verb want, "to be lacking." This definition pre-dated the now more common "to wish for" by 500 years.