See the attached form 1099-DIV and the instructions for box 3.
Based on your limited facts, the amount should be taxable.
If it is taxable, then you would enter this on form 8949 and Sch D just like any other stock sale. It would be taxable in the year the distribution exceeds your stock basis.
Having said that, I don't know the history behind this investment, but it would seem to be limited situations that you would receive a return of capital that exceeded your investment. Just make sure you are confident in your stock basis computation.
https://www.irs.gov/pub/irs-pdf/f1099div.pdf
*A reminder that posts in a forum such as this do not constitute tax advice.
Also keep in mind the date of replies, as tax law changes.
