As well as the IndexOf solution, if you want to stick with the split function then you'd do it like this
string text = "abcd , cdef , efg , ijk , lmn";
string[] values = text.Split(new char[]{',', ' '}, StringSplitOptions.RemoveEmptyEntries);
if (values.Length > 1)
{
values = values.Skip(1).ToArray();
}
text = string.Join(",", values);