Because the amount is over $3000, you can use the Claim of Right (IRC 1341) procedure. If the instructions in the linked article aren't detailed enough, you can post back here with more questions. I believe you need to be using the Desktop program (Turbotax as a download or CD installed on your own Mac or PC). I don't think the needed entry can be made in the online version.
Note that this will only address your federal income tax. If you also paid state income tax on the bonus, you will have to do some research on your own to see if your state has a claim of right procedure and how to use it. Turbotax customer support may be able to help with how to make the claim on your state tax return, but they don't ramp up to full staffing until January.
