Clayton is a boss fought in Deep Jungle in Kingdom Hearts. Sora and company confront him just as he is about to gun down a group of gorillas he had gathered together. Clayton faces Sora in battle twice, first flanked by Powerwilds, and then partnered with the Stealth Sneak immediately afterwards.
In the first stage of the battle, Clayton's only weapon is a shotgun, but he also fights alongside a bunch of Powerwilds. Ignore them and lock onto Clayton, bashing him until his HP is about halfway down, but be warned halfway through his life he will use potions. At this point, the second portion of the battle begins.
When up against Clayton for the second time, the cliff walls explode and Clayton mounts an invisible Stealth Sneak. His only weapon is still a shotgun, which is easily reflected and does not do much damage regardless of being mounted on the Stealth Sneak, so Clayton himself actually poses no true threat, so focus on the Stealth Sneak. Dispose of the Heartless first, then find Clayton and attack him. Sora will have to hit the Stealth Sneak a certain number of times to knock Clayton off in order to attack him. The prize for winning is the Cure spell.
Clayton may heal himself in the middle of a battle. Just deflect Clayton's attacks right back at him, or use the Guard ability. Note that even in the battle with Stealth Sneak, once Clayton is defeated, the battle ends. The Stealth Sneak can be defeated for extra experience though.