Had a horse you could not touch his ears when I bought him. Had to take the bridle apart to put it on. Was okay with clippers everywhere but his face/head area. Started with my trainer giving small carrot pieces while I worked with the clippers on his chin hairs. Then gradually added more head parts - muzzle, bridle path, etc. still with carrot treats.
Found out that when he panics he pulls back and has broken cross ties and flipped when someone tried to clip him for a show. He hates to be restrained. Gradually have worked with him and can now clip ears, head, bridle path with him loose in his stall - do not do show clip on ears except as needed but do run the clippers on his head about once a month just to remind him all is okay. I can clip him in the cross tie area but I do not have him hooked up just in case.
Has taken time and lots of carrot treats but can now clip him anywhere with no sedation and pretty much him just standing there untied or loose.
Carrots work wonders.