set clear-screen

SET CLEAR-SCREEN YES/no  -  whether to clear screen for each message

With the usual YES setting, the screen will clear when MM starts up and
  before each message is displayed.  This makes each message start at the
  top of the screen.

If you change it to NO, the messages just scroll up from the bottom.

You can also clear the screen whenever you want to, by using the 
  BLANK command.

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show   blank   set browse-clear-screen