  The absolute integrated intensity of the single-phonon Raman peak at 1580
cm^{-1} is calculated for a clean graphene monolayer. The resulting intensity
is determined by the trigonal warping of the electronic bands and the
anisotropy of the electron-phonon coupling, and is proportional to the second
power of the excitation frequency. The main contribution to the process comes
from the intermediate electron-hole states with typical energies of the order
of the excitation frequency, contrary to what has been reported earlier. This
occurs because of strong cancellations between different terms of the
perturbation theory, analogous to Ward identities in quantum electrodynamics.
