I would like to be able to display the Build field on the Defect asset. But I am can't see where to achieve that. Am I able to use that field on Defects and not just Stories (Backlog Items)?
No, you are currently not able to add the Build field for Defects directly.
By using the Custom Field feature, you can create a custom field to create one at the Defect level. If you decide to deploy a custom field, be sure to name it something unique, to avoid confusion with duplicates.
If you would like for us to consider adding this as a feature enhancement, please add it on IdeaSpace at - https://ideaspace.versionone.com/default. We will consider it for a future release.