Clustering is currently only supported when you have a web map with a feature layer that has feature reduction. Unfortunately this means clustering is not supported on GraphicsOverlay right now. (It's a mistake on our part that the GraphicsOverlay.featureReduction property appears in the API.)
I will write up an issue internally to record your interest in clustering on GraphicsOverlay. If you'd like to describe your use case a little further, that would help us prioritize it.
