Order entries by custom time field
Craft always saves a complete DateTime value to the database, even if you only make it set / show the time part of it in the Control Panel.

To ignore the date part (which in this case is set to the day you saved your entry) within the order parameter, you can make use of the time() MySQL function:

{% set entries = craft.entries.section('events').order('eventDate', 'time(eventStart)') %}