duty; honor; honour
「一分」 has other readings with different meanings.
武士は一分を守って死を選んだ。
The samurai chose death to uphold his honor.
一分が立たぬと憤る者が出た。
Some were furious that their honor was not upheld.
己の一分にかけて約束を果たした。
He fulfilled his promise on his honor.