This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
焯 combines 火 (fire) with 卓, which likely contributes the sound. The exact historical development is uncertain.
The left side is fire 火, and the right side is 卓 (a shape associated with height or excellence). Picture a bright flame rising high, suggesting brilliance or clarity.
For シャク, imagine a shack lit by a bright fire: shack -> シャク, and the fire 火 makes the shack glow.