Clear : very obvious : not causing or allowing doubt easily understood free from doubt or confusion Plain : having no pattern or decoration not having any added or extra things

Both plain and clear can mean the same thing, "easy to perceive or understand". Plain can also mean "not decorated," "without additional options", "simple," or "ordinary." Plain is also a type of landscape, a large area with few trees, usually covered in grasses. Clear can also mean "transparent" or "see-through".

Depends on the context, could you provide some more info?