This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 斘 is uncertain. It visually combines 夕 and 斗, but the original component relationship is unclear.
Since the meaning is unclear, picture the components: evening (夕) and a measuring dipper (斗). Perhaps it once described measuring something at dusk, but treat this as a memory hook only.
For ショク, imagine a shock (ショク) when you try to measure with a dipper (斗) in the evening (夕) and find the meaning has vanished.