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We address the problem of dynamic assignment of coordination (control) channels in cognitive radio networks (CRNs) by exploiting time- and space-varying spectrum opportunities. Motivated by the inherent grouping of Cognitive Radio (CR) users according to channel availability, we propose a cluster-based architecture for control-channel assignment in a CRN. CRs are grouped in the same cluster if they roughly sense similar idle channels and are within communication range, either directly or via a cluster-head. We formulate the clustering design as a maximum edge biclique problem. A distributed cluster agreement algorithm called Spectrum-Opportunity Clustering (SOC) is proposed to solve this problem. SOC provides a desirable balance between two competing factors: the set of common idle channels within each cluster and the cluster size. A large set of common idle channels within each cluster allows graceful migration from the current control channel should primary radio (PR) activity appear on that channel. Hence, SOC provides a stable network partition with respect to local coordination, with no need for frequent re-clustering. Moreover, when re-clustering has to be performed (due to CR mobility or PR activity), CRs agree on new clusters after the broadcast of only three messages, thus incurring low communication overhead.