What are you talking about? Do you mean first few frequency bins, and setting the others to zero? That would be a simple linear-phase lowpass filter. And if you

What are you talking about? Do you mean first few frequency bins, and setting the others to zero? That would be a simple linear-phase lowpass filter. And if you do the shifting properly then you will see that it has a positive group delay. There are filters with negative group delay, however. But FFT is irrelevant for that and it's not a time machine either.