Mina,
I did not know that the ArcGIS Maps for Office had a cluster option. And you're completely right, this would be extremely useful in AGOL.
I have always relied on custom JavaScript to make clustering happen:
Point clustering | ArcGIS API for JavaScript 3.17
I also stumbled upon this GitHub code for clustering (which also uses JavaScript):
GitHub - Esri/cluster-layer-js: One example of how to cluster many point features
I would suggest making an ArcGIS Idea for this but there are several other ideas out there for clustering and fortunately, this is being considered. You can go here and help vote the idea up:
https://community.esri.com/ideas/6029