First get an arborist, one who is familiar with your locale and can have hands on expert examination. It is obviously an old maple. Everything declines before it dies of old age but while it declines it is very susceptible to disease, insects.
I would take a good handsaw (a narrow pruning saw) and cut that leader (main branch) off right down to the healthy junction, giving an angle to cut so water is not able to sit on the 1" thick stump and forget about coating the stump with anything. Looks like an easy tree to climb but I would still take precautions; no chainsaws, tie yourself to a sturdy branch in case you fall, maybe wear a helmet? If you aren't familiar with cutting down or pruning big trees and branches please hire an arborist to thoroughly check your tree to see if it is worth saving and have them do the climbing and sawing or taking down the entire tree.
Whatever has killed that limb might be contained and this is just part of an old tree. I'd then THIN by removing all small diameter branches to allow light and air to the rest of the healthy tree. Allow wind to flow through unimpeded. The arborist will know...if the problem turns out to be just that branch then you will have more years of use and beauty of this tree. Clean that saw before using and after, using alcohol just in case!