yep, you would be advised to clear the sample especially for any commercial release, the simple fact being that no matter how you came by it, it is still a sample of a copyrighted work being used without permission. True on the demo you maybe less likely to be taken to court, but it does not alter the fact that you would be acting in breach of copyright. if you are at all contemplating a commercial release, and using it as a demo to shop around labels, publishers, venues etc... it is surely better to build upon songs that would not be cause for concern. Better to take the steps now to get it cleared so you can get ahead and make something of the track knowing you have the owner's permission.
Getting it cleared should be a fairly straightforward process.