The defense slot is meant to be filled by a "team defense and special teams unit," rather than an individual player. For example, you might start the 'Baltimore Ravens' or 'Pittsburgh Steelers', in which case you receive points for any sacks, interceptions or fumble recoveries (among other things) accrued by all of the players on those defenses. Plus, if, for example, the special teams unit returns a kickoff for a touchdown, points are awarded for that as well.
However, there are also some leagues that use individual defensive players, which are known as IDP leagues.