First for others reading, if you are looking for an integration document the first place to look is; https://www.arubanetworks.com/clearpassdocs
You probably did that and found out that Red Hat IDM is not listed there.
Given Red Hat IDM appears (I don't have tested or access to it) based on Red Hat Directory server which is LDAPv3 compliant, you should be able to configure it as a generic LDAP authentication source and pull in the user information. As with other LDAP servers, there may be restrictions on access to security-sensitive information like passwords. It may be that you can't configure PEAP-MSCHAPv2 with it, which is deprecated for use anyway.
Depending on how familiar you are already with ClearPass and IDM, you can either start yourself and see if you can make it work, or involve your Aruba partner, Aruba support, or if you have access to that your local Aruba SE to work together and have it documented for other users as well.
If someone is reading this and has worked out how to integrate, please post your experience here.