func clearField(_ fieldno: Int)

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
    dbf.clearField(0)
    dbf.setField(0, asString:"some value")
    dbf.writeField(row, fieldno:0)
    // your code
}