What is the organization called when a group of clans belong to it?

Solution 1:

Are you perhaps referring to the word "Tribe"?

A tribe is normally an organization that consists of many clans.

For example, The barbarians of Icewind Dale were all a large group that lived in the harsh wastes in the North of The Forgotten Realms. Each had their own designation, Such as Tribe of the Elk, Tribe of the Bear, Tribe of the Tiger, etc. And each Tribe had clans that while still belonging to those Tribes, were separate entities.

Those Barbarians were still members of the Larger tribe, they just lived in their own clan and took orders from the leader of their tribe.

Solution 2:

Nation.

Beside 'tribe', also consider 'nation'. The Iroquois League (a.k.a. the Five Nations) was a league of five (or six) nations.

Tribe is not a well-defined term. Sometimes the whole nation is referred to as a tribe, and sometimes subunits of the nation are referred to as tribes.

Each nation was made up of clans.