My mare was never touched with a clipper before I got her...she's 10; she is fine with clipping, but it took 'approach and retreat' type of desensitization. I use cordless clippers on her bridlepath, and facial whiskers, ears, etc...if you have a cordless, that can help with desensitization, as you don't have the cord to have to worry about...though I have an extra battery, and had to use it the first few times, with the desensitization process. Some people have used those battery op. Tooth brushes, to help a horse get used to the sound a clipper makes...I think that is a really good idea, and will probably use that when training other's horses, other than my own.
I also DO NOT tie a horse during this process, as that can cause them to be more clausterphobic and panicked...if she has to move, let her move around you in a circle (again, a good reason to have cordless, though doable with corded too, been there done that.).Start out with the clipper a bit away from her 'problem area, and Keep the clipper on while she moves (don't try to 'touch' her with it, just let her get used to the sound above her eye level), and turn it off, and praise her when she relaxes.
If you are going to use a more 'approach and retreat method', Start out at a place that she is comfortable with, and use that spot as a 'return' area to help her remain relaxed. Try to gradually bring the clipper up to her bridle path, without causing her to move...
I am not sure that treats would help in an instance like that...I don't reward using treats, so I'm not sure about the effective-ness of treating as a reward...you'd have to be 'really quick' in administering a treat, and sometimes you just can't get into your pocket fast enough! Haha! The reward/praise has to come quickly in order for it to 'work', otherwise you're just feeding the horse.