I'd hold the leather over the sink, patch side down. Dampen the patch, spray with a little cleaner, and scrub with a brush. Continuing to hold it upside down, rinse by brushing with plenty of water, and dab dry.
Cleaners can be hard on leather, as they remove the oils. Try gentler ones first, like Dawn dish soap, Woolite, or Oxy Clean. Inorganic stains are virtually impossible to get out completely.
For a new or freshly cleaned patch, mask off and apply Scotch Guard spray protection.