Most horses living in cold climates resemble wooly mammoths at this point in the season. Even if your horse is blanketed, he still may be quite furry. If he's in steady work and will be for the rest of the winter, you may want to clip him, as long as you plan to keep a blanket on him.
Most horses clip just fine. But if your horse has not been exposed to clippers before, he may take exception to cold steel teeth raking his nice, warm coat. Don't be surprised. There are ways to convince him it's not really a giant mutant, buzzing fly.
Horses object to clippers for different reasons. With some, it's the noise; with others, the sight of their hair falling in great tufts confuses them. If the blades are dull, the clipping experience may be painful, making him reluctant to repeat the process. Or he may have received a shock.
It's always better to avoid using restraints such as twitches. Although a restraint may allow you to get the job done once, the experience will solidify the horse's impression that repeating the process is to be avoided. It's much better to eliminate his anxiety.
It's important to determine exactly what it is about the process that frightens him so you can deal with it. Here are a few suggestions:
The key here, like most unfamiliar activities for equines, is patience and making the procedure as pleasant as possible. If you are gentle and persistent, most horses are quite willing to learn.