The best defense against a Muay Thai knee is to beat the entry. If your opponent has to step forward to load that knee, you fire a jab or a teep into their chest before they get there. They are on one leg. They cannot eat a straight punch or a straight kick in that position without paying for it.
That is the whole trick at long range. Tip, jab. Tip, jab. Every time they lift that leg they put themselves in a very bad position, and the straight weapons beat them to it.
The nightmare is when you are already in the plum and they have broken your posture down. Now you are stuck. From there, cross your hands up in front of your face to shield, and start alternating your legs to defend the knees as they come — one side, then the other. Do not camp there. You are losing every second you stay.
As soon as his attacking leg comes back down and he resets, that is your window. Drive your head forward, shoot both arms in low, and get double under hooks on his lower back. Grip low and tight. From that grip you can dump him or at minimum rip yourself out of the plum and escape to open space.
One detail coaches miss — the under hook has to be low. Hands at the belt line, elbows crushed in. Up high on the shoulders you are weak and he will turn you. Low on the hips, you own his base.
That is the whole answer. At range, beat him to the punch with the jab or teep. In the clinch, shield, wait for the reset, and dive in for low double unders.
For the full breakdown of clinch control, posture, and counter-attacks from the plum, check out our full guide to Muay Thai clinching. The complete course lives in The Ultimate Muay Thai Training System.