Take it apart - get a new gasket (or O ring). Clean everything up and reassemble using "good stuff" (a gasket sealer).
Make sure the bearing housing is laying flat against the axle housing end. Most people just beat the brake backing plate flat- bad idea...I prefer to clearance the housing it self since this gives it a better seal.
Regardless of what will be posted, c-clips can be leak free IF you take the time and care to install them correctly (most people don't)