clear
皙 centers on a clear, bright, and unblemished appearance, especially of skin or complexion. The single given meaning 'clear' points to this visual purity and lightness.
皙 combines 析 (split wood) with 白 (white), suggesting the idea of something split open to reveal a clear, white interior. The exact historical development is uncertain, but the component combination fits the meaning of clear or bright.
Imagine splitting a dark log (析) to reveal the pure white wood (白) inside. That clear, bright inner surface is 皙.
For シャク, picture a shack in the woods where a woodcutter splits logs to reveal the clear white wood: shack -> シャク, and the split wood shows the bright 皙.
white (complexion); fair-skinned