For now, you could actually take out the MATE clock applet and install indicator-datetime, trying to pull as little of Unity/GNOME as possible:
sudo apt install indicator-datetime --no-install-recommends
Then add its executable to Start-up Applications:
/usr/lib/x86_64-linux-gnu/indicator-datetime/indicator-datetime-service
It works OK and puts itself to the left of the power button like Unity, except that as it’s not integrated to MATE, you’ll need to dig in dconf-editor to modify settings (show seconds, day of week, etc)