Removing empty/low amplitude part from signal
If you know the frequency of the sine signal  already, you can correlate your [.......p-set...] [SINE-WAVE] [....q-set...] signal with the sine signal. The correlation will give you a peak where sine wave exactly matches with the signal. Since you know the sine signal length you can figure out the beginning of the sine signal in your original signal. I hope this helps you a bit.