Well, not being a native english speaker does not help for this but to me, "aggression" is not a quality that relates to tackling skills but more to the ability to win duels. In my country, a defender who tackles is a bad one. Thiago Silva, Laurent Blanc or Paolo Maldini (fantastic defenders according to this definition) were very good in aggression but hardly ever tackled. Good defenders are always well positioned, remain on their feet and win duels. Tackling is left to the others..