Libgdx Mouse just clicked

See http://code.google.com/p/libgdx/wiki/InputEvent - you need to handle input events instead of polling, by extending InputProcessor and passing your custom input processor to Gdx.input.setInputProcessor().

EDIT:

public class MyInputProcessor implements InputProcessor {
   @Override
   public boolean touchDown (int x, int y, int pointer, int button) {
      if (button == Input.Buttons.LEFT) {
          // Some stuff
          return true;     
      }
      return false;
   }
}

And wherever you want to use that:

MyInputProcessor inputProcessor = new MyInputProcessor();
Gdx.input.setInputProcessor(inputProcessor);

If find it easier to use this pattern:

class AwesomeGameClass {
    public void init() {
        Gdx.input.setInputProcessor(new InputProcessor() {
            @Override
            public boolean TouchDown(int x, int y, int pointer, int button) {
                if (button == Input.Buttons.LEFT) {
                    onMouseDown();
                    return true;
                }
                return false
            }

            ... the other implementations for InputProcessor go here, if you're using Eclipse or Intellij they'll add them in automatically ...
        });
    }

    private void onMouseDown() {
    }
}

You can use Gdx.input.justTouched(), which is true in the first frame where the mouse is clicked. Or, as the other answer states, you can use an InputProcessor (or InputAdapter) and handle the touchDown event:

Gdx.input.setInputProcessor(new InputAdapter() {
    public boolean touchDown(int screenX, int screenY, int pointer, int button) {
        if (button == Buttons.LEFT) {
            // do something
        }
    }
});