Its a healthy looking clump!.
Those tight one are tricky to divide. You can try storing it whole and dividing in the spring, or dividing now and storing the tubers separately. Stored whole, you should probably cover it in slightly damp peat moss or vermiculite in a container and keep it as cold as you can without freezing it. If your basement is heated, your garage might be better, unless it gets really cold.
This is a good article on dividing and storing
http://cubits.org/Dahlias/arti...
