Too light primordial black holes evaporate and are therefore strongly constrained by various bounds, e.g. Cosmic Microwave Background distortion. However, if they are formed strongly clustered, the corresponding haloes may collapse into heavier black holes which may form the entirety of the dark matter of the universe. The indirect signal of such a scenario is the production of a flat stochastic background of gravitational waves which is detectable by the Einstein Telescope.