This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
粆 combines 米 (rice) with 少 (few, little), but the exact historical development and original meaning are uncertain.
With no clear meaning, picture a small amount of rice: 米 (rice) and 少 (few) together suggest a tiny portion, but treat this as a visual mnemonic only.
For サ, imagine a small (少) sack of rice (米) labeled with the sound 'sa': sack -> サ, tying the shape to the reading.