Sidearm Pocket is a gear perk that slightly raises how much sidearm ammo you carry. It is a small boost for players who lean on a pistol or other sidearm.
You hold a little more sidearm ammo, so you can use your pistol longer before switching back to your main gun.
Wear the Munitions Pants in the legs 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 your sidearm does real work. Most players get more from a defense or speed perk in the legs slot.