Combine butter and flour in a saucepan until butter is melted and a you have made a roux (thick butter/flour paste). Set aside.
Dice potato, celery, onion, leek, and bell pepper; toss all into a large saucepan. Chop clams and add to veggies. Combine all remaining ingredients except half-and-half.
Simmer until potatoes are thoroughly cooked.
Add butter-flour mixture to chowder and stir until thick.
Remove chowder from heat.
Stir in half-and-half until blended.
Heat to serving temperature, stirring occasionally.