I've inherited a SP Online site and would like to create a new site collection to replace the existing collection. Is there actually a way to do this? Or will I need to just tweak the existing site?
I think you mean the root site collection which is a little different in that you can't recreate it once it's deleted.
You'll need to open a support ticket with Microsoft to do this. It might take some time - these guys are kind of slow (as in, "methodical") but they'll be able to do that.
you have two options
1.- Request a new site
2.- Clear your current site
you've the ability to delete and create site collections when needed, please check this link for the steps to create a new site collection and also the steps to delete a site collection
you must be either a Office 365 global admin or SharePoint admin
You can create a new site collection if you are a global admin . IF you want the content of existing site collection , you can backup the content of this site collection using file explore and copy to your new site collection .