We analyze the influence of clustering on the collision rate of inertial particles in a smooth random velocity field mimicking the smaller scales of a turbulent flow. The dynamics is dependent on two relevant parameters: the compressibility of the flow and the ratio between the relaxation time of the particle velocity and the correlation time of the field. For small values of the ratio, clusters cause smaller collision velocities independently of the flow compressibility. For large values of the ratio, clusters cause smaller or larger collision velocities depending on whether the random field is compressible or not. An interpretation in terms of the geometric structure of the clusters is proposed.