If your SSA-1099 is showing a 2017 payment along with your 2018 benefits, then the 2017 payment is considered a lump sum payment. You enter it into your return and TurboTax will calculate whether any of it is taxable or not.
Take a look at the TurboTax article below for more information and guidance to enter this into your return.
https://ttlc.intuit.com/replies/4767454
