I want a kit that only clan leaders can use it.How can i do it?

Well you can't do that at the moment. You need to make a plugin that will give permission to clan owners or make special edit in kits plugin

can you help with this?

Until a plug in is made (see plugin request page) you can create a group, compare it to the leaders shown in the clans data file, and manually assign them to the group. Allow that group a custom kit.