Alex,
My SEQCROW plugin adds a selector for everything on the same fragment as the current selection. It only looks across bonds, not pseudobonds (e.g. metal coordination or H-bonds). The selector is `connected`, so you'd run `select @{atom_name}; select connected`.
If you want to do this interactively, it also adds a mouse mode that lets you click on a fragment to select it. You would need to restart ChimeraX after installing for the mouse mode menu options to show up.
Plugins can be installed via Tools -> More Tools... on the ChimeraX menu.
Best,
Tony