Clam Chowder comes in many shapes and forms, the most common ones being Manhattan clam chowder (made with tomatoes) and the so-called New England clam chowder (with milk and cream used as a base). If you are thinking about losing weight, the one you should avoid is New England clam chowder.
Most of the ingredients used for preparing Manhattan clam chowder are very healthy, such as onion, garlic, celery, and tomato. However, the sauces used should be added in moderation. And what really makes this meal good for you are clams themselves - of course, you need to go for the fresh ones, as canned ones are much richer in sodium, which you want to avoid (it raises your blood pressure and helps your body retain water)
Most of the seafood is rich in heart-healthy fats, and clams are no exception; they have more protein than chicken and have more iron than beef, that's what makes Clam Chowder such a good choice for a healthy diet.