How should I prepare a whole (already roasted) leg of lamb bone for stock?
It is good to roast the bone before using it for stock in order to get color on it. Color adds flavor, so it is worth it. Only the bone exposed in the oven in the original roasting will have gotten any color as the rest was covered in meat. Don't worry about cleaning it completely, a bit of meat here and there isn't going to hurt your stock.