This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 閞 is uncertain. It combines 門 (gate) with 弁, but the original function of this compound is unclear.
The gate 門 encloses 弁, a shape associated with a valve or a cap. Imagine a gate with a special valve-like mechanism, but its exact purpose is a mystery.
For ヘン, picture a hen pecking at the mysterious gate valve: hen -> ヘン, and the hen is curious about the strange 閞.