Tunnel Fighter

You excel at defending narrow passages, doorways, and other tight spaces. As a bonus action, you can enter a defensive stance that lasts until the start of your next turn. While in your defensive stance, you can make opportunity attacks without using your reaction, and you can use your reaction to make a melee attack against a creature that moves more than 5 feet while within your reach.

(This is intended to be a Fighting Style that a fighter gets at level 1.  If you wish to use this fighting style then take this feat and leave the fighting style selection blank.)