Leaning trees are a professional's problem.
Call a certified arborist. Not your buddy with a chainsaw.
A tree leaning on another tree is under extreme, unpredictable tension. Cutting the wrong side can release that energy instantly — the tree can kick back, roll, split, or swing sideways. Even experienced loggers get killed doing this. The physics is nasty. The leaning tree is storing a lot of potential energy, and you don’t know exactly where it will go.
There are techniques — using rope, winches, wedges, or cutting in a specific sequence — but they require experience and the right gear. One wrong cut and you’ve got a tree through your roof or your ribcage. This is not the time to watch a YouTube video and “figure it out.”
If you absolutely have to do it yourself, at least have a plan for the tree to go exactly where you want it (and a backup plan). But honestly? Just pay the pro.