+1 to what mikelat said. If there happens to be more than one L fare in the market you're looking at and really want to force just that fare basis, you can use:
from: CLE:: / f UA.CLE+PSP.LEK14SFS
to: PSP:: / f UA.CLE+PSP.LEK14SFS

You can also uncheck the "Check seat availability" checkbox to see what dates the fare is valid for, then restrict your search to available seats after that.

But the first part of the fare rules...
...should answer your question. (You can pull up the fare rules by clicking "Show booking details" on ITA when looking at a particular itinerary and then looking for the "rules" link next to the fare basis.)