The clearinghouse is the part of the NCAA that reviews a student's eligibility for scholarships. That can involve issues with grades, educational requirements, or issues of amateur violations.
As for what the issue was, unless there is more info it could be a wide variety. Simple issue of poor grades would have been known up front I'd guess. I've heard some athletes get tripped up because of questionable "summer classes" that don't quite meet standards. Amateur violations can be weird. Nick Kerdiles was dinged for a suspension partially because of allegations an agent paid for plane tickets.
If he returned to the EJHL then it means it's something that can hopefully be resolved, maybe by taking another class or something. Unless he goes to the CHL, he'll be back.