How do I trim only the selected clip on a timeline with a shortcut key?

If you left-click on any individual clip with a mouse--that will select only that clip. Usually you need an initial selection of any one clip on the timeline before using shortcut keys. Then you can use keys like:

Select Previous/Next Clip (Up/Down Arrow Keys)

..To select individual clips using shortcut keys. To select individual edit points, you can always use things like this as well:

Select Nearest Edit Point as Ripple In

Select Nearest Edit Point as Ripple Out

Select Nearest Edit Point as Roll

Select Nearest Edit Point as Trim In

Select Nearest Edit Point as Trim Out

Once again, many amazing features in Premiere remain unmapped and require you go into Keyboard Shortcuts and map them.