With the current setup, you need to get the value from clock.restart() and divide it by 1.f / deltaTime.
Then you can use std::to_string(fps) to convert it to a string, which you can then draw with a sf::Text object after loading an sf::Font object.
If this is all a bit much, then I recommend to keep reading on, you'll eventually learn most of this in the book.
Btw. I believe the processEvents should only exist once in your code.