This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 竫 is uncertain. It visually combines 立 (stand) and 争 (conflict), but the original meaning and component relationship are unclear.
With no clear meaning, focus on the shape: 立 (stand) beside 争 (conflict). Imagine standing firm in a conflict, but remember this is just a visual mnemonic, not a real meaning.
For セイ, imagine standing (立) in a conflict (争) and saying 'say' (セイ) to calm things down. The word 'say' cues セイ.