Mathf.Clamp Problem with Character Controller Script

Can anyone help me please as I am trying to use the character controller script to move my character whilst restricting its movement beyond the scope of the camera. Help much appreciated.

var speed : float = 0.01; private var moveDirection : Vector3 = Vector3.zero;

function Update() {

 var controller : CharacterController = GetComponent(CharacterController);
 moveDirection = Vector3(0,Input.GetAxis("Vertical"),0);
 moveDirection = transform.TransformDirection(moveDirection);
 moveDirection *= speed;      
 controller.Move(moveDirection * Time.deltaTime);

Debug.Log(moveDirection); moveDirection.z = Mathf.Clamp(moveDirection.z, -1.0, 6.0);

}