In your home directory, a file called .bash_profile or .bashrc should be there.
In this you could write at the bottom:
If you did this, typing "cls" would do exactly the same as "clear"
You could also use symlinks. Depending on where the command "clear" is, you could use the command:
Which would symlink "clear" to "cls"
Either way would work.
Hope that helps,
Kourosh