Answer: Based on your description, it sounds like the branch is about to break and split away from the tree. If this is truly the case, you could cut it off to prevent it damaging the main trunk. To do this, begin with an undercut from the bottom up to remove the weight and bulk of the branch, then make another clean cut next to the branch collar -- the slight swelling where the branch joins the trunk. Do not damage the collar, cut just outside it. The collar is what will grow out and over the cut to seal it shut. Do not use pruning paint.
This is a fairly drastic recommendation, so I would hate to just tell you to do that sight unseen. Instead, I would strongly suggest you take a few photos of it -- both a closeup of the branch and one of the overall tree -- and consult with your county extension and/or trained professional nursery staff about what to do.
If the tree is a large and valuable one, or in a prominent position in your landscape, I would suggest you consult on site with a professionally trained and certified arborist with experience in ornamental landscape trees. They should be able to determine the initial cause of the problem and long term prognosis for the tree as well as the best way to handle the current crisis.
Best of luck with your tree -- I'm sorry it is in trouble.
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