This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
晠 combines 日 (sun) with 成, but the exact historical development is uncertain. It may have been intended to express brightness or completion related to the sun, but this is speculative.
The sun 日 beside 成 suggests something becoming bright or complete, but since the meaning is unclear, picture a rare character where the sun is trying to become something—its meaning is still forming.
For セイ, imagine the sun saying its own name: 'Say' (セイ) as it tries to become bright. The sun's voice cues the reading.