smoking weed in small, enclosed areas, such as a car. It's called clambaking because when you're done smoking and you open the windows, all the smoke pours out, just like clambaking

Apparently, people from Connecticut call hot boxing clambaking. In the words of a certain CT resident “its smoking pot in your car, rolling the windows up...like a clambake.” It combines the wonders of first hand and second hand smoke to maximize THC intake.