If I'm understanding you correctly, you received a distribution from an insurance policy. The gross amount is appearing in box 1, box 2a is blank, and box 5 has a figure less than the figure in box 1.
Since the box 5 figure reflects the amount of insurance premiums you're allowed to recover tax free in 2016, that figure will be subtracted from box 1. The difference will be the taxable amount on your tax return, and it'll appear in box 16b of your Form 1040 (12b of 1040A).
I don't see that you'll need to change anything. If a box is blank on the 1099-R, leave it blank when you enter it in TurboTax.