Non-negative matrix factorization (NMF) based sound source separation involves two phases: First, the signal spectrum is decomposed into components which, in a second step, are clustered in order to obtain estimates of the source signal spectra. The major challenge with this approach is the accuracy of the clustering algorithm in the second step, especially as most previously used clustering algorithms are focusing on the frequency part of NMF only and, hence, are missing the information of the time activation matrix. In this paper, we propose a novel clustering criterion which combines the frequency and time activation part of NMF. It is based on the linear predictive coding compression error and we show that it allows a good clustering of the NMF components while at the same time can be efficiently computed. Our new clustering criterion shows an overall improved performance compared with the current state-of-the-art clustering algorithms as we experiment on the TRIOS dataset.