set browse-clear-screen

SET BROWSE-CLEAR-SCREEN YES/no  -  whether to clear screens during BROWSE

With the usual YES setting, the screen clears before the prompt for the
  next message when you use the BROWSE command.  

If you change it to NO, the prompt for the next message will appear at the
  bottom of the current message text.  (The screen-clearing before a message
  is displayed is controlled by the variable CLEAR-SCREEN.)  You will need 
  to set it to NO if you also SET BROWSE-PAUSE NO.

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show   set browse-pause   set clear-screen
(at the MM> or Read> prompts:)  browse