If you are doing 892.1x with clearpass, there are two parts:
1. Ldap lookup to find the user
2. 802.1x authentication with AD.
For #1 you need to create your own redundancy for the ldap lookup of the user. You should enter a backup IP address specifically for that or just put the fqdn of the domain for clearpass to choose a server randomly.
For #2 the request is just sent off to any available domain controller so in a way there is built in redundancy.
I hope this makes sense.