This paper introduces an amendment to radio interferometric calibration of sources below the noise level. The main idea is to employ the information of the stronger sources' measured signals as a plug-in criterion to solve for the weaker ones. For this purpose, we construct a number of source clusters, with centroids mainly near the strongest sources, assuming that the signals of the sources belonging to a single cluster are corrupted by almost the same errors. Due to this characteristic of clusters, each cluster is calibrated as a single source, using all the coherencies of its sources simultaneously. The obtained solutions for every cluster are assigned to all the cluster's sources. An illustrative example reveals the superiority of this calibration compared to the un-clustered calibration.