"xià" is a measure word and "yí xià" is often used after a verb as a complement of frequency, meaning that the time that an action takes is short or that it's easy to do something, e.g.,
a. děngyíxià 等一下
a.這本書我看了一下，很有意思。
zhè běn shū wǒ kàn le yí xià ， hěn yǒu yì si 。
C. 這首歌很簡單，你稍微練一下就會唱了。
zhè shǒu ɡē hěn jiǎn dān ， nǐ shāo wēi liàn yí xià jiù huì chànɡ le 。