Removing a clock mainspring barrel
Often a clock mainspring barrel can be removed without disassembling the clock movements outer plates. The mainspring will also still be contained within the barrel.
Letting down the mainspring power
First you would use a mainspring let down tool. This device allows you to hold the tension of the mainspring with your fist rather than your fingers. Put the mainspring let down tool over the square post the key goes onto. With a flat screwdriver, move the "click" out of the way. The click has a tension spring that needs to be held back. This is what holds the mainspring so that it winds in only one direction. Slowly allow the mainspring to unwind in your fist. This method lets you control the speed of the unwind with your whole fist rather than your fingers. Unquestionably, doing this with a key will result in a painful experience on the fingers. Of course, if the mainspring is broken, there usually is no worry since there isn't much tension to begin with.
After the barrel is out
Once the power has been let down, the next step is to yank out the winding arbor shaft. The clock mainspring barrel should then fall out the side of the movement. After the barrel is removed, you will be able to see the number that is on the barrel end cap. This will allow you to determine what hole end mainspring to order. You can locate the mainspring by using the hole end mainspring chart. Be sure to check the movement for any damage as a result of the mainspring breaking. In particular, inspect the teeth of the gears, as well as the arbor on the next wheel up. This could have consequently been bent.