* "lesson" - This is the closest sound-alike, but it has a different meaning entirely, referring to a learning experience or teaching.
* "lessen" - While this is the same word, it can be pronounced slightly differently, with a more distinct "n" sound, making it sound similar to "lesson" for some speakers.
Ultimately, finding a true homophone for "lessen" is difficult. If you're looking for a word that sounds similar and has a related meaning, you might consider "reduce" or "diminish."