What to Do: General Tips
Having confirmed someone is indeed trying to undermine you, the challenge lies in what to do next. Introspect. The reason may be some misunderstanding or spite owing to some genuine shortcomings on your part. A subordinate, colleague or your boss might have looked upon you for help and you remained oblivious and did not respond. If so, apologize and resolve to make amends. The issue may also have to do with personality differences rather than any competence or specific issues. Understand such differences, identify whatever common ground exists, and cope the best way possible.
Help others and set standards in competence. Incompetence breeds politicking. Highly competent people and team players remain popular, and people rarely backbite against such popular figures. If they do, the backbiter themselves becomes unpopular, for the grace and competence of such starts never remain in doubt. Keep away from rumor circles and unnecessary socializing beyond what is needed. If the office is a minefield of politics, keep work life separate from social life. Concentrate solely on work at the workplace, leaving socializing and friends outside work.
Never reply in the same coin, or get aggressive. Spreading lies or counter-accusations leaves no difference between you and the accusers. If matters get worse, the company would simply punish both of you. Getting aggressive or physically assaulting the person, regardless of the provocation, can reverse the tables, and make you the aggressor. Keep any confrontation civil and maintain calm. Unfavorable power equations, such as the accuser being too close to your boss, might render you helpless at times. Bide your time, using the delay to prepare a strong case against the accuser. Revenge is a dish best served cold!