Answer
Clarke 1866 is a spheroid, rather than a datum, and no transformations are available from any spheroid to a datum.
If the data is defined as D_Clarke_1866, the datum may be D_North_American_1927, because Clarke 1866 is the spheroid on which the NAD 1927 datum was constructed.
Further research is necessary, however, to determine the correct datum, so the projection for the data can be correctly defined, and projected and transformed to another coordinate system.
For guidance in this process, refer to the related articles below.