How to check if a geometry is contained in a geometry collection in postgis?

How can I check if a geometry is contained in a geometry collection ? Specifically , I am using ST_ClusterWithin in PostGIS to get a cluster which returns each cluster as a geometry collection. Now I want to extract the cluster_id for each geometry. For this I need to check if the geometry is within the geometry collection. How can I do it ? It is related to this question I asked on stackexchange some days ago.