Succour us out of the city - David, with a reserve, would hold the city, and either support the bands in case of need, or receive them within the walls should they be compelled to flee.
Related Commentaries of 2 Samuel 18:3
2 Samuel 18:3
3 But the people answered, Thou shalt not go forth: for if we flee away, they will not carea for us; neither if half of us die, will they care for us: but now thou art worth ten thousand of us: therefore now it is better that thou succour us out of the city.