This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
爝 combines 火 (fire) with 爵, which likely contributes the sound. The exact historical development is uncertain.
The left side is fire 火, and the right side is a complex shape 爵. Without a clear meaning, picture a torch made from an ornate vessel: fire held in a ceremonial container.
For シャク, imagine a shaky torch flame: shaky -> シャク, and the fire 火 flickers unsteadily.