You have to evaluate the Left function first. How you do that depends on how you produce your return separated list. Some forms would require a script or recursive custom function to do that. In cases where the list is from a related table, however, you may be able to set up a calculation field that returns the left N characters of one field and then you use your List function to list that calculation field.
This list was generated via ExecuteSQL So I guess i would try to put left around that first?
You would have to put the left function inside the SQL expression such as:
"Select Left ( \"fieldName\" , 10 ) ...