Heat bacon grease in a cast iron Dutch oven over medium heat; cook and stir potatoes, carrots, onion, and garlic until onion is soft, 5 to 10 minutes. Add flour, salt, garlic powder, liquid smoke flavoring, and black pepper; stir until vegetables are evenly coated.
Pour chicken broth and reserved clam juice over vegetables; cook at a simmer over low heat until potatoes and carrots are tender, 15 to 20 minutes.
Melt butter in a saucepan over low heat; stir in cornstarch until smooth, 1 to 2 minutes. Add milk and cream; cook, stirring constantly, until thickened, about 5 minutes.
Stir butter mixture into vegetable-broth mixture; cook and stir over low heat until chowder is thickened, 5 to 10 minutes. Add corn and cook for 5 minutes. Mix chopped clams into chowder and cook until heated through, about 5 minutes more.
