You'd need to do:
If you just do car(TheForm->fieldList), you'll get the symbol which is the name of the field. What you were doing was setting a property on the symbol, not on the form field. So what I'm doing is getting the the form field structure itself, and then updating the value.
Note I would generally recommend against using fieldList - that's all very well until you then add a button or something else onto the form. Better would be to ensure that your fields are named deterministically by using something like concat('CLfieldThing count++) rather than gensym, and then record the number of fields on the form using TheForm->myFieldCount=count, or record the fields that you generate using TheForm->myFields=listOfFields. Note you can add whatever properties you like on the form data structure, so it's useful to store related information.