You could left justify the definitions in the glossary using the HTML editor this will take precedence over what is set in the theme. This would be a lot of work if you have a lot of terms and subject to missing a few.
If you want to change the default that may depend on the theme, but I do not think you can change just glossary pop ups. If you change the default it will affect other dialogues in moodle. Which theme are you using?