If you need to remove a pesky spot stain, for something like pen ink, then rubbing alcohol can be effective. Use a cotton swab dipped in rubbing alcohol to spot treat stained leather.
Be sure to test it on an out of the way spot, to ensure that the alcohol has no adverse reaction to the specific leather you’re treating. Alcohol can dry out and crack leather, so be sure to only use it for hard to clean spot stains. Always be sure to rub off any excess fluid, let dry completely, and protect the leather once cleaned with a leather conditioner.