You can call 'getEditor' method to get the current editor and then get the combo object.
onCellEdit: function(index,field,value){
var input = $(this).datagrid('input', {index:index,field:field});
if (input){
if (value != undefined){
input.val(value);
}
var ed = $(this).datagrid('getEditor', {index:index,field:field});
var cc = $(ed.target);
console.log(cc)
}
}