When you preface a statement with the phrase "In [someone's] defence", it means that you are going to make an excuse for that person's behaviour in the next thing that you say. For example: Someone says "Richard's so dramatic! One moment, he's laughing, one moment he's crying. It's exhausting." You might reply. "I know what you mean. But, in Richard's defence, he is an actor. Actors are often like that." I hope that makes sense.