Boxing/kickboxing can help tone that area (especially if you can power the punches from the lats), but as Fit Mommy said, you can't choose where you lose.
Reducing bodyfat overall will help (the 'bra overhang' area is one of the places that women tend to store excess fat on the upper body).
It can help to burn more calories and fat by doing full-body weight workouts, or at least weight workouts that focus on the larger muscle groups of the legs/glutes, back and chest (rather than smaller shoulder/bis/tris).
Another thought: as you add more activity, your low calorie count may work against you. If your body is not getting sufficient calories (or, especially, nutrients), it can hang on to the fat you want to lose.
HTH!