Clean out the area with a knife or something, then paint it with Arbrex (from the garden centre) - this sounds like gummosis, caused by some kind of damage to the trunk. You can buy some tape to stick over the top, using a stick or something to keep the stem straight if its actually bending or splitting as well, though I've used a roll of sticky plaster for this (the kind we use for cuts) and its been okay. Remove it eventually though.