The
official tutorial explains enough about sf::Clock and sf::Time usage.
You should use sf::Clock only once (initialize at a start of project) and you use sf::Time to manipulate the values you receive from sf::Clock.
As said in the tutorial, the main usage of sf::Clock and sf::Time is to update the game logic:
sf::Clock clock;
while (window.isOpen())
{
sf::Time elapsed = clock.restart();
updateGame(elapsed);
...
}
Please note that there are other, better ways to update game logic, like using fixed time steps. But if you are just starting out, the code above should be enough.
Also in this case, it would be smart to use vsync or to set the framerate limit.
You should also read (and bookmark) this:
http://gafferongames.com/game-physics/fix-your-timestep/ - it is really helpful, especially when you want to start with more complex projects.