Once, For A Moment, Temporarily
adverb
You only have time to do one thing at dawn once. Dawn is really short so you can do whatever it is for a moment, but that's it.
いったん
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
この話は一旦なかったことにしてもらえませんか。
Will you forget that conversation for a moment?