What is the relationship between clustering and association rule mining?
First of all, the problem of finding frequent itemsets (ARM) differs from the similarity search (clustering). 

If you think about market baskets, clustering tries to find objects that have a large fraction of their baskets in common, the absolute number of those objects (basket occurences) is not of interest.
With ARM we are interested in the absolute number of baskets that contain a particular set of items.

CLustering:  Allocates objects in such a way that objects in the same group (called a cluster) are more similar (given a distance metric) to each other than to those in other groups (clusters).

ARM: Given many baskets (could be actual supermarket baskets) find which items inside a basket predict another item in the basket.

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