Nice idea.
But there's no need to make it just for clans. I think a lot of clans would be interested in keeping score in a competitive thing. But I think it would be nice to not just limit it to clans.
How about also a non-clan orientated ladder? Make it like the native mixed 5 a side thing. So people form a group and give it a stupid name and then fight. And people can advertise for players, or advertise themselves as a player.
So you've got professional football, and 5 a side pub teams - two different competitions. It would need two different ladders, but it would allow clanless players to also form a team and fight, and also allow people who are in clans to get a different experience and be able to play with people who aren't in their clan - many people have friends in other clans, or who aren't in clans etc. Maybe add a rule like in native 5 a side that you can only have a limited number of same clan people in one team.
If it's just one for clans, then clanless people won't be too interested, and there are only so many clans that could even form a 15vs15 team.