A feud is essentially a declared war between two clans. A clan you are feuding with is guaranteed to be a thorn in your side.
Although the Riders, Rams, and Wheels all treat feuds somewhat differently (for example, the Wheels call them oaths of enmity, and Riders and Rams have trouble ending feuds because they don't share customs to do so), Ride Like The Wind glosses over most of the differences.
At the beginning of Ride Like The Wind, your clan is not feuding with anyone, although other clans do have opinions of you and some may already hate you.