  The cosmic 21-cm background frequency spectrum related to the spin-flip
transition of neutral Hydrogen present during and before the era of
reionization is rich in features associated with physical processes that govern
transitions between the two spin states. The intervening electrons in
foreground galaxy clusters inversely Compton scatter the 21-cm background
spectrum and modify it just as the cosmic microwave background (CMB) spectrum
is modified by inverse-Compton scattering. Towards typical galaxy clusters at
low redshifts, the resulting modification is a few tenths milli-Kelvin
correction to the few tens milli-Kelvin temperature of 21-cm signal relative to
that of the cosmic microwave background black body spectrum. The modifications
are mostly associated with sharp changes in the cosmic 21-cm background
spectrum such as due to the onset of a Lyman-$\alpha$ radiation field or
heating of neutral gas. Though low frequency radio interferometers that are now
planned for 21-cm anisotropy measurements are insensitive to the mean 21-cm
spectrum, differential observations of galaxy clusters with these
interferometers can be utilized to indirectly establish global features in the
21-cm frequency spectrum. We discuss the feasibility to detect the spectrum
modified by clusters and find that for upcoming interferometers, while a
detection towards an individual cluster is challenging, one can average signals
over a number of clusters, selected based on the strength of the
Sunyave-Zel'dovich effect at high radio frequencies involving CMB scattering
alone, to establish the mean 21-cm spectrum.
