It depends where you want to store you data. If you want to make a spectrogram, I suggest you to store FFT data in a matrix (jit.matrix) using jit.peek~ to store and jit.poke~ to read the data. Working with FFT data is sooo much easier and more intuitive when the data is stored in a matrix that I really suggest you to go in that direction.
J.F. Charles published an awesome article in Computer Music Journal back in 2008 called A Tutorial on Spectral Processing Using Max/MSP and Jitter. There are also many accompanying patches to that article...have a search on the web. Priceless source of information!
PS: And you will have to decide between using fft~ or pfft~. There is no point in using both. At least not for what you want to do.
PS2: In case you're not familiar with Jitter, you really need to know just few basics in order to create Spectrogram, so...I would say Jitter is the way forward:)
If you really want to isolate just a single bin (as opposed to displaying the entire spectrum), this may not be the most efficient way imaginable, but it works. (The example uses fft~ instead of fftin~, but the principle is the same.)
You can use ==~ and sah~ to accomplish this. (as in when the bin# is equal to n, sample the value and continue holding it until the next frame. You can use send~ on this. You may need to divide by the number of overlap since you're adding multiple values together)