# Smoothing peak spectra in time domain

I would like to smooth amplitude spectra of signals around their peak frequencies. I could perform FFT, find the peak frequency, and smooth around the peak in frequency domain, then inverse FFT back to time domain. However, is there a way to do the spectrum smoothing in time domain? If finding peaking part is difficult, is there a time domain operator that smooths spectrum for a given frequency band? What would be the theory behind?