(I'm actually not sure whether it would technically be unsuccessful under the rules framework, but to answer your question to the fullest I assume not.)
It would not be successful because a character can only participate once. There is no change in game state by making a character that is already participating, participate a second time.
So if you use Ours is the Fury on a character that is already defending, there is no change in the game state from the first part of the ability. There is a change in the game state, though, by creating the conditional "stand" effect. This is what allows you to play the card in this situation -- beyond the fact that the already-defending character matches all the targeting requirements - the game state changes by creating the lasting effect.
That's important because the rules say that if using an ability does not change the game state in any way, you cannot use the ability. (Grimwalker is correct that if the word "then" appeared between the "defend" and "maybe stand" parts of the ability, you could not play event; the failure of the "defend" part would "then" stop the "maybe stand" lasting effect from being created, and there would be no change in the game state - making it illegal to trigger the ability.)