You get a 'defend kill' by killing an enemy that is trying to capture your flag. You need to get five of these kills to unlock the achievement.
You will likely get this achievement early on just by playing the game but if you are having problems try the following.
When you are on the respawn map screen, look for a allied flag where there are a couple of enemies nearby or heading towards it. Choose this flag and you will spawn in the area, now find a hiding place and wait for the enemy to start capturing the flag (look for your flag falling on the pole), now kill them. Repeat five times and the achievement will unlock.