This is not a gnome-clocks bug, i think you should dicuss this directly with gnome-designers, we just implements their designs.

I know, but as there's no gnome-design product, and I don't use IRC, report on application is the only mean I have to report the issue I found.

Thanks for your work.

Could you please check the latest gnome-clocks mockup? [1]

I feel that it would reduce the number of clicks required to delete an alarm, as you have a simple close button for each alarm, dismissing it.
I know that's not a solution for all the gnome products using the selection mode selection pattern, but is a good start for clocks.

[1] https://raw.githubusercontent.com/gnome-design-team/gnome-mockups/master/clocks/alt/clocks-wireframes-2.png

Clocks 3.38 has a delete button at the right of every item in the list. It is now a single click action.