I can't see a way that you can delete a time entry once the green plus symbol has been clicked.
After you have rejected a pending timesheet the time entry still exists.
Hope you find this message well.
Please visit this how-to article to learn how to add/edit time entries in nTask : https://support.ntaskmanager.com/support/solutions/articles/43000061747-add-task-effort-to-timesheet
To remove a time sheet, simply insert all '0's in the time entry column.
If you have any other questions, please feel free to write to me at firstname.lastname@example.org. I'd be more than happy to help.