What is the collective noun of cluster?

There isn't really a specific collective noun for "cluster" in the same way there is for "flock" of birds or "herd" of cattle.

The word "cluster" itself implies a collection, so it's often used directly to describe a group:

* A cluster of grapes

* A cluster of stars

* A cluster of buildings

However, you can use more descriptive terms depending on the context:

* A constellation of stars

* A bouquet of flowers

* A clump of trees

* A swarm of bees

* A hive of activity

Ultimately, the best way to describe a cluster depends on what you're trying to convey.