This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
鶒 combines 勅 (likely phonetic) with 鳥 (bird). The exact historical development is uncertain, but it appears in the word 鸂鶒 (けいせき), a type of water bird.
The left side 勅 suggests an imperial command, and the right side 鳥 is a bird. Imagine a rare bird summoned by decree, though its exact meaning is unclear.
For セキ, picture a secretary (セキ sounds like 'seki') recording the sighting of this rare bird: seki -> セキ.