How serious is a wide cut excision to remove a lesion on back of leg?

Nothing trivial: No operation is trivial, yet this type of procedure does not involve any major organs. Depending on the extent of the resection, local anesthesia may be sufficient which would mean that general anesthesia could perhaps be avoided.

Depends: It depends on how serious the 'lesion' is -- if this is a benign lesion, i.e. No cancer, than it isn't serious. If it is a malignant lesion ie. Cancer, then it can be serious, e.g. Melanoma. As long as you have a good surgeon, the technical aspects of removing a lesion on the leg are not that difficult.