You have to be careful with this as there are two simliar sounding phrases that mean different things. "To have [someone] in your pocket" means you have complete control over them. This is frequently used in football (soccer) games e.g. "The defender had the attacker in his pocket the whole game" meaning the defender was in complete control of the attacker "To have [something] in your BACK pocket" means you have the option of using that thing (which can be a person) in a situation where it is perhaps unexpected but will give you an advantage. Similar to the phrase "To have [something] up your sleeve" - an example would be "I'm giving a presentation to these Japanese executives this afternoon - if I really want to impress them, I've got Bob in my back pocket who speaks fluent Japanese"