Why do I have a high peak at the beginning of my FFT (not DC)?
You cannot simultaneously have a non-zero value at the first bin of the DFT (the 0 component) and a zero-mean since the first component is by definition of the DFT the (scaled) mean of the signal:
from the DFT:
$$X[k] = \sum_{n=0}^{N-1}x[n]e^{-j2\pi nk/N}$$
$$X[0] = \sum_{n=0}^{N-1}x[n]e^{j0}= \sum_{n=0}^{N-1}x[n] = N\mu_x$$
So there is a mean in the digital data, and perhaps the OP's confusion is there is not a mean of any significance in the analog waveform being sampled. There are several opportunities to introduce offsets notably in the ADC converter itself, or it may be as simple as the mapped amplitude range of the digital waveform includes an offset. But taking the mean of the data that is being FFT'd should be consistent with what is above, with further scaling by any windowing that may be done according to $\sum_{n=0}^{N-1}w_n$.