Time Series pattern recognition and classification problem
I do not fully understand what you mean by "average frequency". But this may help to think about it.


You can calculate the power spectrum of the signal using DFT. 
From the power spectrum, you will be able to get the average power of all the frequencies.
Use this value of average power to map the frequency which it closely corresponds to. 


This will give a sort of the average frequency you are looking for from the average value of power.