The Clan Castle is considered a resource building, so Goblins target those. But if you're suggestion a troop that singly targets that building, there will be no point to them. If it is a tank unit like the Lava Hound, meaning a lot of health, it would deal low damge. If it is a high damage troop, low health. Or if for instant it has both high damage and high health, it's speed may be a factor, not to mention the housing space required to hold one of that troop. Regardless, a good base would have their Clan Castle in a protected area, so if that troop ever reaches the area, it could be shot down fairly quickly.