How does the top look? That wound looks like it was from last year. The wood is weathered grey and the callus tissue has begun to heal over. Maybe a buck rub, maybe a frost crack..., in any case it looks nasty but if the top looks healthy it will probably eventually (after a few years) heal over. Do not apply any paint or pruning sealer or anything. If the top is showing signs of stress, such as branches dying back, I would probably just replace it, especially if it is a specimen tree in your yard.