Help needed: How to make a library from a .ino-file?

Using Arduino Programming Questions
8

Some of the methods in the class should probably be private, instead of all of them being public.

sendData() is a method the belongs to a specific instance of a class. As opposed to a static method that is shared by all instances of the class.

Therefore, to call it from Clear(), just use the method name. The ClassName:: bit up front, called a scope resolution operator, is not needed.

void EDIPTFT::Clear()
{
  char command [] = { 12 };
  sendData(command, 1);
}

The space key is not important to the compiler, usually, but it does make for more readable code.

There are some other things you could do. command does not need to be an array to hold one value. You could use:

char cmd = 12;
sendData(&cmd, 1);

But, if you wish to keep it an array, for consistency with other functions where the array contains more than one element, you should use the sizeof() macro, and let the compiler do the counting:

sendData(command, sizeof(command));

That way, if you copy/paste the function and change the size of the array, you don't need to count the number of elements in the new array.