Not an emergency, so please don't use the emergency tag. Thank you!
As far as the question, they are technically compatible, as in neither is likely to harm the other. However, clown loaches get big. Think over a foot long under the right conditions. Given that they also need to be kept in schools, I wouldn't recommend any less than a 125 gallon. A betta will be ok in such a large tank, but it will need lots of plants (either live or silk, avoid plastic) in order to provide cover. Some of these plants should go up near the surface as bettas, being anabantoids, need to breathe air. I will say that clown loaches need more flow than bettas, and more alkaline water, so while the combination is possible, I wouldn't really recommend it. Hope this helps, good luck!