Tank Commander is a gear perk that gives you extra Supremo charge for kills you make while driving a tank. It lets you build up your Supremo faster during heavy fights.
Each kill from inside a tank adds bonus charge to your Supremo. You can roll out of the tank with your Supremo ready to use.
Wear the Soviet Cap in the head slot. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
Only if you often take tanks. It has no effect outside of tank play.