It is the rootstock. The new growth has trifoliate leaves and start below the graft line. If top part is dead, you can remove the whole tree and reuse the space.

I agree on its rootstock. If you break the rootstock growth off there is a chance that your trees main trunk my be alive and send out shoots. You can always rip it out later.

6b Steve

Yes, the green shoot is the rootstock - but it doesn't necessarily mean the rest of the tree is dead. After a damaging frost, it can take some time for new growth to begin again. The rootstock is hardier and will tend to grow first. As the others say, you can remove that shoot and wait to see what happens. If you scrape away a little of the bark higher up the tree and find there is some green under the surface, then there is still hope.