Engineering and controlling well-defined states of light for quantum information applications is of increasing importance as the complexity of quantum systems grows. For example, in quantum networks, high multi-photon interference visibility requires properly devised pure photon sources. In this paper, we present a theoretical model for a spontaneous parametric down conversion source based on an integrated cavity-waveguide, where single narrow-band, possibly distinct, resonant modes for the idler and the signal fields can be generated. This mode selection takes advantage of the clustering effect, due to the intrinsic dispersion of the nonlinear material. We show that, by engineering the clustering effect in an integrated cavity-waveguide and by using a standard detector, one can efficiently generate heralded pure single photons even with a continuous-wave pumping mode. The photon source proposed in this paper is extremely flexible and could easily be adapted to a wide variety of wavelengths and applications, such as long-distance quantum communication.