Here wedges are your friends. You'll need at least 4. The tree is being supported so you cannot "fell" it. What you can do s use your ck to pull it off it's stump once you make cuts to free it. Make two felling cuts on opposite sides of each other one on the lean side and the other opposite and parallel to the ground. I usually cut the lean side first. Leave about a 3"-4" hinge. Bang in 4 wedges into these cuts as you are going along: 2 on one side, two on the other about a foot apart for an 18" tree. Plunge cut or continue cutting the hinge out. The wedges will prevent the tree from crushing the chainsaw bar. The tree should now be free of it's stump and because the cuts are parallel to the ground, your tractor should easily pull it off it's stump. I've had big leaners slide off the stump all by themselves but these are like finding pearls in oysters.
Now here's what to watch out for. Some trees will want to twist off their stump which is fine because you are never standing in back of the tree to be the brunt of a possible deadly "kick out". As suggested, use a long cable and or pulley snatch block as sometimes, as Coby already stated, a tree will grab it's stump and the top will start moving toward the pull line especially if the tree is not leaning very much. A long enough cable coupled with a perpendicular pull and a low cable set, will mitigate this being problematic. I've taken down hundreds of "leaners" this way with few problems. Start with smaller trees until you get the hang of it but I usually reserve this method for larger trees. For that 6-12" tree, I use the following method:
Make a felling "notch" opposite the lean side about 3'-4' up. Make the felling cut. The tree will collapse on itself. Keep cutting like this until the rest of the tree hits the ground or pull the rest of the tree off with the tractor.
If you cannot get a tractor in there, a 2-4 ton come along can be used.