This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
熚 combines 火 (fire) with 畢, which likely contributes the sound. The exact historical development is uncertain.
The left side is fire 火, and the right side is 畢. Without a known meaning, picture a rare, flickering fire that is hard to name.
For ヒツ, imagine a hissing fire that makes a 'hitsu' sound as it burns: hitsu -> ヒツ.