Can a timesheet entry can be deleted once it's entered?

The short answer is no, you can't delete posted job lines, whether it is a timesheet, resource or material entry.

To remove a transaction from a job you will need to cancel the transaction on the Job Line Processing tab of the master job.

This will reverse all the transactions associated to the line. Then you will need to re-enter the line with the correct information.

If the incorrect information on the jobline is an incorrect Subjob ID, then the line can be moved to the correct job. Again, go to the Job Line Processing tab of the job, right click on line and select Move Line. A box will pop up prompting you to enter the correct Subjob ID and posting date.