Is it possible to collapse the first accordion within the Side Accordion App?
No it's not possible without custom development. Note that we are releasing an hosted version of the Side Accordion template with ArcGIS Online release tomorrow. See A sneak peek - the new Story Map Series
not sure what you mean? That the first panel disappears completely?
Yes. So when you open the app, all the panels are collapsed.
It appears that the Side Accordion app hosted on ArcGIS Online is defaulted to having the first accordion set as "open" and there is no way around this.... BUT, you could get the downloadable version and host it on your own server. This involves a little coding but its not too bad. I actually had to do it for my Shortlist story map Shortlist
Here is a tutorial that may help you on your way: Story Map Side Accordion Tutorial | Story Maps
Retrieving data ...