When a horse is frightened of an object, the worst thing you can do is force him to approach it. The more you say, "You WILL accept the clippers," the more nervous the horse gets and thinks that there is a reason to be scared of it. If you want your horse to use the thinking side of his brain and relax rather than reverting to the reactive side of his brain and moving his feet, you have to stop being the predator that takes a direct line approach, and start thinking more like your horse--a prey animal--that is cautious. Rather than forcing the object on him, you’ll use a step-by-step method to desensitize him.
Whenever you desensitize your horse to any object, you'll always use the Approach and Retreat Method. You'll build his confidence by approaching him with the object he is scared of and then retreating by taking the object away when he stands still and relaxes. It's important not to take the object away if the horse is reactive and wants to move his feet. Mother Nature says, "Are you crazy? Don’t stand there, run! If you're frightened, don’t hang around, run!"
You're trying to say, "Don’t listen to your mother. When you get frightened, stand still and relax." You want the horse to realize that the quickest way to get a scary object to go away is to ignore it. For example, using the Approach and Retreat Method you can teach your horse to stand still and relax while you clip him, apply fly spray or flap his body with a plastic bag. You'll teach him that if he stands still and relaxes, the object will go away.