from my understanding gaping hole on clams is bad. keep an eye on it. IMO thats way too open to be healthy.
found this info on nano reefs
Clams have an intake siphon and an output siphon. The intake siphon looks somewhat like a mouth and is a slit opening that is found on the center line off to one side of the mantle while the output siphon looks like a cone or rod. A clam is considered to be gaping when the intake siphon is open very wide almost like a tear. Indication of gaping vs. normal feeding would be not only that the intake siphon is open abnormally wide but that the mantle isn't extending normally and the shell is fully open. Gaping is caused by poor lighting, parasites, or poor health and if gaping continues the clam will not recover. Treatment should be examination for parasites followed by moving the clam to a higher or lower level in the tank depending on its proximity to the light source.
http://www.nano-reef.com/topic/153218-a-faq-on-clams/