That's what I was thinkin' sir_dudeguy. Maroon's are very aggressive, and several other species of clown CAN be (Tomato, Two Stripe, etc.). But chasing a mandarin is pretty odd behavior for an Ocellaris.
On the right track their sammyp. Tank size would make a large difference in the territorial behavoir of the clown. Even an ocellaris can be territorial.
Is the clown kept singly or is he with a mate? Ocellaris tend to do better in pairs. Mine get along great with my mandarin....well, neither pays the other any attention anyways. In fact, my Ocellaris tend to swim with my regal, yellow wrasse, 6 line, scooter blenny, and used to hang out with my coral beauty until it jumped tank on me. Their about as freindly as i've ever seen a fish be.
Does your mandarin try to eat the food during feeding times? If so, I'd consider getting rid of the mean clown for two new ones and keep the mandarin you have. It's not often they can be weened onto prepared foods or pellets.