How to filter out everything but a single frequency in the time domain?
Depending on the exact characteristics of the signal and implementation requirements (if any) I can think of a number of approaches to extract content at a "single" frequency. The most common:


Apply an FFT to calculate the spectrum of the signal and extract the frequency of interest.
Apply Goertzel's algorithm to effectively extract signal content at a single spectral line.
Apply a bandpass filter around the frequency of interest.
Apply an inverted notch filter around the frequency of interest.


Depending on the amplitude and frequency content of the signal, the additional noise present and other implementation requirements (e.g. should the signal be processed in real time or offline, should the implementation be digital or analog, etc.) a specific approach might be preferred.