the following code taken from lazarus/examples/grids/gridcelleditor relates to what you want to insert a radio button in a string grid, substitute the radio button for the combobox.
procedure TForm1.StringGrid1SelectEditor(Sender: TObject; aCol, aRow: Integer;
var Editor: TWinControl);
begin
if (aCol=3) and (aRow>0) then begin
ComboBox1.BoundsRect:=StringGrid1.CellRect(aCol,aRow);
ComboBox1.Text:=StringGrid1.Cells[StringGrid1.Col,StringGrid1.Row];
Editor:=ComboBox1;
end;
end;
Regards
Dave