The only way you can safely disassemble a clock with an open mainspring is by using mainspring restraining clamps. 4 piece sets, which will accommodate various mainsprings sizes are available from all clock suppliers.
Wind the mainsprings tightly enough so that you can work the clamps around the springs. Move the rachet click aside and allow the let-down key to release the mainspring slowly into the clamp.
With the mainspring safely clamped you can proceed with cleaning and servicing a clock movement.
When a clamp may not fit or you do not have clamps, a soft steel wire will suffice.
Round and flat mainspring clamps are available from suppliers. The above shows a flat clamp on a mainspring
If you can afford it, a spring winder is one of several essential tools of a clockmaker.
Safety is paramount when working with a clock. If you are going to buy just one set, I would purchase flat clamps. They are easier to maneuver around the mainspring than a round clamp. Mainsprings are not to be fooled with. They are very powerful and all that power can cause damage not only to clock components but to your limbs as well.