There is no such thing as spot training, so to see more muscle definition you'll have to cut your bodyfat in general.
Exercises like v-ups, dragon flags, v-sits, ab-rollouts, good mornings, and back extensions will help you develop your core.
Jaw size is dependent on genetics. Of course, you can help your genetics along by squatting and deadlifting heavily and release more testosterone and growth hormone. These will make the jaw bigger and fuller. Or, you can supplement these chemicals and take steroids. I don't recommend the latter.