Hi,
the most common FFT algorithm is limited to a input sample count of 2^n (where n is an integer number). --> 2, 4, 8, 16, 32, 64...
Is your algorithm able to use other input count values?
if I understand correct, then you want to perform an FFT, pick only the 1kHz bin and perform an IFFT. The result is just the 1kHz signal that is included in the input signal.
If so, then why don´t you use a simple FIR filter. It consumes less processing power and less other ressources like RAM...it runs in real time (sample in, result out, not block wise like the FFT)
Klaus