IPA: /əˈlɜrtli/
KK: /əˈlɜrtli/
In a way that shows you are paying attention and ready to respond.
She listened alertly for any signs of danger.
Alertly → It is formed from "alert" (from Latin "alertus", meaning to be on the watch or attentive) and "-ly" (a suffix meaning in a manner of). The word "alertly" means in a manner that is attentive or watchful.
Think of being 'attentive' or 'on the watch' ('alert') and how the '-ly' suffix indicates the manner of doing something. So, when you act alertly, you are doing so in a watchful way.