This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 鏁 is uncertain. It visually combines 金 (metal) with 巢 (nest), but the original semantic and phonetic roles are unclear.
With no clear meaning, picture a metal (金) nest (巢) — perhaps a locked metal cage or a chain, but treat this as a visual placeholder only.
For サ, imagine a metal nest that is sealed shut with a sharp 'sa' sound: sa -> サ.