If you get hit in the varicose vein and a big bump forms, is it a clot?

Maybe: Varicose veins are abnormal veins and prone to certain problems. If hit over a varicose it might develop superficial thrombophlebitis. This is an inflammatory reaction with thrombus of a vein under the skin. It could also bleed under the skin. Either one could form a "bump". Treatment is conservative but if it persists, don't hesitate to get it checked out.

Couple possibilites: Trauma can injure the wall of the vv, and this weakens the wall. The pressure within the vv dilates this injured area, and a big bump is the result. Injury of the vein, with stagnant flow, and abnormalities in the blood (virchow's triad) makes that area susceptible to forming blood clot. If the big bump is firm, and not collapsable when pressure is applied, it is a clot.

Could be: I agree with drs. Isobe and thomas, you certainly could have a clot. Varicose veins are often chronically inflamed and if you hit one, it can become more inflamed. More inflammation can certainly make the possibility of developing a clot more likely. If you elevate your leg and the bump doesn't go down in size and become softer, you should get it checked out.