It will disappear when line comes up with a DTE cable attached. When the interface is to be used as a DCE you will be able to remove it (using: no clockrate) as well once the link is active.
If it accepted the command that implies that it is connected to a DCE cable. What happens if you connect it to DTE cable. In my experience that removes the clock rate command.
Or have you tried the command default clock rate? I am not sure but I think that should set it back to default which is no clock rate.
