Ted,
You need to bend the tabs out to get the back cover off. Take your time and go gently with that. Keep in mind, you need to bend those back in place to hold the clock face and back cover together. You also need to remove the terminal from the back.
Now you can get to the clock works.
Use some light air pressure to blow dirt and dust out of the clock works, caned air for cleaning computer parts works well.
Then you can spray a little WD40 on the clock assembly followed by some light air pressure.
Look around the clock works and find the electrical points. Gently pull the points apart and let it return on it's own. At this point, if the mechanism is in working order, it will run till the points close. Typically these clocks stop working due to the points getting burned. I file the points with a "points file" , I think it is older then I am. haha You can clean the points with fine sand paper, it will take longer though. Probably 240 grit followed by 320 grit paper should do. The finer the point surfaces are, the longer the clock will work.
Now you can test the electrical part of the clock. I use two wires with alligator clips on them and a 12 volt battery. Attach one wire to the frame of the clock assembly and the negative terminal of the battery. Now you can connect the other wire to the terminal point on the clock and the positive battery post. If you have cleaned the points well, the clock will start working with a "snap". It will run on the spring mechanism as the points slowly close and again "snap" it starts all over again.
I usually let it run for a few hours before putting it back together.
Good luck,
Loren