It can be simple
A league system would probably work fine if we had the following simple rule (as an example): Your clan has to play all other clans in the same division once during one season (which would be 7 weeks with 8 clans in each division), when you win a match you gain 1 point, when you loose, you loose 1 point. When the season ends top clan in each division reach a higher division and get as many points as the clan with the least amount of points has there. Bottom clans drop down one division and get as many points as the clan with the most amount of points has there....
If 7 weeks is too long time for you to try to reach a higher division, then we could have 5 clans in every division, and each season would then be 4 weeks long...
As soon as a clan sign up they are put in a division (and thereby required to play 1 match every week...)
The bottom division can have less than 5/8 clans but if they become 10/16 clans the division is just split in 2.