A few things.
How long have you had this star?
If less than one month, how long did you acclimate it?
Have you recently fed it, or noticed anything go missing?
Has it lost any arms?
What sort of fish, shrimp, crabs do you have?
If there is simply a big hole through the disk, and the arms are still intact, and the brittle is otherwise normal, just let it be. Make sure nothing will pick at it, like shrimp. If this is a problem, you may need to put it in a small critter cage or something in the tank. Do not remove it to another tank at this time. Moving it within the system, to a sump or refugium, is OK. Otherwise, the acclimation will stress it.
I had a brittle get a hole in the disk last week. Amazing. My current theory for sudden huge holes in the disk is called distended disk syndrome, in which the animal eats something that is too big or sharp, and it punches through the disk. This appears to be fairly common in certain species.
A healthy brittlestar will repair this damage, if it is not too bad, in remarkable speed. I would say that mine had a hole covering more than half the back of the disk. This was about 1 week ago. It is now closed, though he is still acting a bit shy.
It was really gross- you could see right through it and out the mouth, but from the inside. Yikes.
This should not be a problem at this time. Just make sure tank parameters are good and stable (what is the specific gravity, BTW?). Feed him if he wants it.
If the arms start to drop off, this is a more critical situation. Monitor closely, but I wouldn't do anything further at this time unless it is being bothered by other animals.
In this case, as mentioned, consider a plastic critter cage (put in the tank) or a hang on breeder trap with a small bit of rock.