to level off, to trim, to pare down
剗 centers on cutting or scraping away excess to make something level or neat: trimming, paring down, or leveling off a surface.
剗 combines 戔 (which likely contributes the sound and the idea of cutting or damaging) with 刂 (knife), reinforcing the action of cutting or trimming.
The left side 戔 suggests damage or cutting, and the right side 刂 is a knife. Picture using a knife to trim away damaged edges until the surface is level and smooth.
For サン, imagine trimming a piece of wood with a knife under the sun: sun -> サン, and the knife pares down the wood evenly.