This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
賋 combines 貝 (shell, money) with 交 (exchange, mix). The exact historical development is uncertain, but it likely relates to financial exchange or clarity, given the reading あきらか (clear).
With 貝 (money) and 交 (exchange), imagine a clear financial transaction where money changes hands openly, making everything あきらか (clear).
For コウ, picture a clear financial exchange where coins (コイン) are used: コイン -> コウ, and the transaction is transparent.