Can I select an area of a clip and be able to modify it?

Oops - should be available now.

Look for things using opacity masks and/or track mattes. A track matte is used based on some element in one layer to show or hide something on a higher layer. Essentially.

To really start using this program, it takes some time to learn which pays off big when working after it. I suggest getting Jarle Leirpoll's book "The Cool Stuff in Premiere Pro" which sounds like a light thing but is 500+ PACKED pages of practical editing from project management, basic to very advanced cutting steps, an amazing array of keyboard shortcuts and the reason to set and master them, sound, Lumetri color, motion graphics and animated titles including a bit of working with After Effects links, and a ton more.

It's an ebook available from his site of premierepro.net, and there's a lot on his site besides.

And past that, a few months with hours spent working through Lynda.com tutorials is fantastic.

Also retooled.net and premieregal.com have some stuff ranging from basic to well above.

Neil