Peter,
I also had the same problem when using RSX11M instead of M+.
Below is a quick Macro-11 program that sends two VT100 escape sequences.
The first clears the screen and the second sends the cursor home.
It can be easily modified for other terminal types if needed.
For command line editing that works with RSX11M, take a look at the MCE command line
editor.
It is a great DECUS program found on RSX SIG Tape Spring 1989 in [265,1]
Best,
Mark
TYP CLR.MAC
.TITLE Reset VT Terminal
.MCALL QIOW$S,EXIT$S
.EVEN
MSG:
.ASCII <33>/[2J/<33>/[H/
LEN = . - MSG
.EVEN
START: QIOW$S #IO.WVB,#5,#1,,,,<#MSG,#LEN,>
EXIT$S
.END START
MAC CLR=CLR
TKB
TKB>CLR=CLR
TKB>/
TKB>TASK=…CLR
TKB>//
PIP LB:[1,54]/NV/CO=CLR.TSK
INS $CLR