Loose adult tooth. How do I get it anchored down?

You need care. You need a periodontist to determine the cause of the problem and offer treatment options.
Not sure if can. Why is it loose? Trauma or just getting that way? If no trauma then go see your dentist and likely you have periodontal issues with that tooth and may not be able to get it anchored.
Sign of Gum disease. A loose adult tooth, if not resulting from direct trauma, is usually sign of gum disease or traumatic occlusion. The proper diagnosis need to be made and a comprehensive treatment plan discussed with the patient. Please seek your oral health professional for care.