When I introduce a horse to clippers, they are not tied. That way they have room to move and get away and won't pull back. I first turn them on and just let them run. From there, I'll slowly move it towards their shoulder, and then place the clippers on their shoulders and just let them vibrate and make the noise. When the horse is comfortable and confident, I move them slowly up the neck. I'll just slowly introduce them and then carefully clip. It really helps when another person is there to assist you!
Best of luck!