You can delete a draft through below steps depending upon the application you are using:
iOS:
Swipe to left on the draft that you wish to delete and you will receive the option to delete the draft.
macOS:
Right-click on the draft that you wish to delete to receive the option to delete the draft.
Android:
To delete a draft, long-press on the draft and you will receive the option to delete it.
Windows:
Right-click on the draft that you wish to delete to receive the option to delete the draft.