First of all, Charles Clay is NOT a pure fullback. He has played running back, fullback, and tight end. http://sports.espn.go.com/ncf/player/stats?playerId=237445 911 career rushing yards, over 2,500 yards receiving, and 38 total touchdowns.
Now all we can hope is that the Dolphins can find a position for him, where he will be effective as an offensive player for the Dolphins. It appears he is a good player at several positions but not a great player at any of them. That is probably why he was still available in the sixth round.
He appears to be a great receiving player, whether those receptions come from running back, fullback, or tight end.