void clearField(cdbfapiPlus* handle, int fieldno);

The function clearField() removes all data from a certain field and fills it with zeroes (0x00) or spaces (0x20).
You can call this function before calling setField(), setMemoBuf().

{
    // your code
    clearField(dbf, 0);
    setFieldString(dbf, 0, "some value");
    writeField(dbf, row, 0);
    // your code
}