How do I run Start() inside a nested class?

Start is one of the functions Unity will call, not you.


What are you actually trying to accomplish here?

Just access the instance of the subclass in your $$anonymous$$onoBehaviour's Start method, and call the subclass' Start method from there.

If your nested class is not a $$anonymous$$onoBehaviour (and it should not be), then to initialize the class use a constructor, or call an initialization function explicitly. For example:

 public class Data_access : $$anonymous$$onoBehaviour {
 
     private void Start () {
         Fronts f = new Fronts(); // either let constructor initialize, or...
         f.Init(); // ...explicitly call a public initialization function (you can name it whatever you want), but make sure it is only called once
     }
 
     public class Fronts {
 
         public Fronts () {
             // do here all the initialization you need, that you would usually do in Start()
         }
 
         public void Init () {
             // or do it in an explicit init function
         }
 
 }

Note, that a constructor is more like Awake() (because they are both called when the object is instantiated), and Start() is more like an initialization function (because they are called before the object is updated for the first time).