Self-Defense: Use when a hostile figure enters a space adjacent to you. That figure suffers 1 damage.

So if I displace a hostile figure with a massive figure, can I play this card since they are entering a space adjacent to my massive figure?

Sure, as long as the Pushed figure does enter a space adjacent to the Massive figure. (It is possible all adjacent spaces are occupied or invalid and the figure is Pushed to a non-adjacent space. Observe the correct order to Push figures: first the figures controlled by the owner of the Massive figure, then the others.)

Sneaky use of a defensive card!

Effectively gives Massive Vehicle figures 3 "trample" cards in addition to Overrun and Crush, unless I am missing another one.