An elder is someone in a clan who has many rights. Clan elders can promote members to elders, can invite people in the clan (by sending "mail"), and most importantly... Kicking. But elders cant kick elders, only leaders can. I made this blog for all clan leaders to be careful who to promote to elder, they can kick many members. I have a past experiences of this. Someone joined our clan and kept asking and asking for elder. Our clan leader finally gave him elder and then everyone left. The next morning on our clan chat I saw everyone was kicked. I scrolled down all the way and it kept going. I checked how many people we had left and it was 5. It was me and the elders left.
So I highly recommend that you know the person you want to promote before you promote them. Anyone could just be playing with you and when you promote them, they will kick everyone. I have seen many clans who have everyone as elder, it's a smart idea so nobody can kick anyone, but nobody gets any rights.
I hope this helps you in the future of promoting.
(I know this has been put in on Clan Politics but I wanted to touch on this subject a little bit more.