This word does NOT mean taking down a fence; it is the British spelling of "defense" — a word that means the act of protecting or defending. Wearing garlic around your neck might be your defence against vampires.
Defence can be verbal as well as physical. If you're in court charged with a crime, you (and your lawyer, if you have one) are called "the defence." If your only defence is that you were sleepwalking when you robbed a gas station, you might be going away for awhile. If you play defence in a team sport, your main job is to keep the other team from scoring (so stay wide awake).