This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
偰 combines 亻 (person) with 契, which likely contributes the sound. The exact historical development is uncertain.
A person 亻 beside 契, a shape associated with a contract or pledge. Without a clear meaning, picture a person bound by an ancient agreement.
For セツ, imagine a person setting a seal on a contract: set -> セツ, and the person 亻 is beside the contract 契.