A long time ago (in a galaxy far, far away), I read a sentence in a book (can't remember which one) that has stood me in good stead through the years.
"Perception is all there is."
Because this person perceives you as defensive, then anything you do to argue that you are not will only convince him further. Becasue this person can affect your career, you need to change his perception through your actions, not through argument.
First consider that he may be right. Senior managers are often very perceptive where people are concerned. Really look at your interactions with this person as if you were in his position and it was one of your subordinates taking the same actions or saying whatever it was you said to him. For instance, did you dispute one of his decisions after it was made? This is an extremely bad idea in virtually any work environment. The time to bring up objections to decision is before it is made, not after. Did you behave as if you knew better than he did and that you weren't going to do what he decided? That's even worse! Did you act as if he hadn't heard your objections the first 5 times you told them to him? Did you waste his time, presenting the same information mutiple times?
Next consider that he and you may be in different generations and thus perceive the same actions very differently. Different generations are often as much alien to each other as actual aliens from another planet would be.
But whether it is objectively true or not, he perceives that you are defensive and felt so strongly about it that he told you or told your manager to tell you. So you have to take the same actions whether you actually were defensive or not.
First apologize for whatever it was that got him upset. It doesn't matter if you still feel you are right, you need to apologize. Do not stick your foot in your mouth still further while apologizing. "I'm sorry if you were offended" is not an apology and will only make things worse. It is best to apologize in person, so if he will be at your office anytime soon, wait until then and make an appointment to see him. If he will not, then calling him and talking over the phone is the next best idea. You will probably want to make an appointment to talk though. Email is the third choice.
Next, ask him for help in appearing less defensive. Tell him you never intended to appear defensive and you need to learn better communication techniques so that it doesn't happen again and ask for recommendations as to how to do that. This will tell him that you heard his criticism and you are trying to fix the problem. A defensive person is not big enough to admit to error or to try to fix a problem. This alone will go a long way in correcting his perception of you.
Rehearse what you are going to say (or draft the email) and how you are going to say it and have a friend critique. The very worst thing you can do at this point is to sound defensive, so find someone who will be ruthless about criticizing you as you practice. Then after you are sure what you are going to say is fine, have someone who knows this person fairly well such as your boss, critique you.
Now for future behavior. You probably know what action it was that caused him to call you defensive, so do not repeat that action. In any interaction with him, ask yourself if this could be construed as defensive and think before you speak and then think a second time. Especially accept criticism well and do not try to defend your actions if he criticizes. Say something like, "I can see your point. What would you suggest?" And listen to the suggestions and do them.
