This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
蔶 combines the grass radical 艹 with 責, which likely contributes sound. The exact historical development is uncertain.
The top grass radical 艹 over 責 suggests a plant or something made from plants, but the exact meaning is unclear.
For サク, imagine a sack made of woven grass: sack -> サク, and the grass radical 艹 ties it to the kanji.