Often, when using clocks, newbies to SFML tend to stick them in the wrong places and restart them at the wrong times. Things like that can result in "funky" behavior at best.
Keep in mind that every line of code that isn't empty or commented out takes time to execute. Depending on how a function that is being called, or a class that is being constructed, is implemented, the time value might range from miniscule to infinite.
Things like updating all of the game entities in the world, performing calculations, and rendering are fairly computationally expensive, so make sure to not somehow exclude these calls from the span of your time measurement. Always make sure that restarting the clock and grabbing the elapsed time is the