Collins Chinese Dictionary Plus (3rd edition), 2011:
一下 yīxià
I 量
- 我去问(問)一下。 Wǒ qù wèn yīxià. I'll just go and ask.
- 请(請)大家自我介绍(紹)一下儿(兒)。 Qǐng dàjiā zìwǒ jièshào yīxiàr. I'd like everybody to introduce themselves.
- 这(這)件事我们(們)还(還)得商量一下儿(兒)。 Zhè jiàn shì wǒmen hái děi shāngliang yīxiàr.
We still need to discuss this.
measure word, used after verbs to indicate one's attempts to do something
II 副 at once
- 天一下就冷了。 Tiān yīxià jiù lěng le. All at once the weather turned cold.