With the steadily growing number of service providers the competition becomes more and more intense. In order to find a distinctive edge over other competitors, automatic service composition can be applied to further adapt to the requirements of the users. Most of the current composition approaches can be categorized as either planning or selection algorithms. The former automatically create workflows and tend to neglect Quality-of-Service (QoS) concerns, drawing compositions useless if user constraints are violated. Selection algorithms on the other hand optimize the QoS but provide no functional flexibility. In this paper we show how the strengths of both approaches can be combined by leveraging common characteristics of service registries. Therefore we utilize a data structure which arranges functionally similar services in clusters and computes the QoS of each cluster. Our planning tool composes workflows consisting of these clusters, taking the QoS of the clusters into account. This way, the utility in general and the reliability of the composed workflows are significantly increased. We prove the feasibility and the benefits of our functional clustering in our detailed evaluation.