It sounds like the top corner of the tank is hosting your clownfish. My young ocellaris does the same thing, stays in on corner of the tank. It is most likely normal behavior as long as it is eating normally, not breathing rapidly, and its appearance is normal (no spots or anything).
Is the fish gulping at the surface for air or just swimming under the surface?
How long have you had the fish for and how long has this behavior been going on for?
What size tank and what kind of flow do you have, how many powerheads/what kind?