Foot op - due to broken bone, then wore a cast, but after diagnosed with clot in sural vein. doctors removed the cast to treat. is this advisable?

Yes: Last thing you want is a clot in your veins that can dislodge and travel to the lungs and cause pulmonary emboli. That is more important than a broken foot for sure.

It sounds: Like the prudent thing to do. If you have a clot, you don't leave the cast on. As long as you are not walking on the foot, i.E using crutches, the bones should be okay.