one
「一」 has other readings with different meanings.
one; 1
most; best
一声かけてから入って。
Say something before coming in.
一口だけ味見させてくれない?
Could you let me have just one taste?
一晩寝たら気持ちが落ち着くかもしれない。
If you sleep on it, you might feel calmer.