If the domain's home page is to redirect to the /amp/ subdirectory, what is the purpose of not using the root directory for the /amp/ - or it is only for creating a /subdomain/ that is not available as a domain.tld? You can send all traffic to your subdomain but whether you can and whether you should are two different questions that can depend on the purpose of the plan.
The How-to part of that is dependent on using server configuration or .htaccess to manage.
reading between the lines, it sounds like you have an amp version of your site set up in the /amp/ subdirectory of the main site and you now want to access the amp site using a subdomain.
if this is the case, i would suggest making the home page of the amp site sub.example.com/ rather than sub.example.com/amp/ in order to do this, you must configure another virtual host for the subdomain which specifies the /.../amp/ directory as the document root directory.
i would also suggest configuring a 301 redirect so that requests for resources on the main domain with a url path starting like /amp/ should get redirected to the canonical url on the amp subdomain.