Popular Spinner

Create an adapter

public class ExampleSpinAdapter extends ArrayAdapter<Estado>{
// Your sent context
private Context context;
// Your custom values for the spinner (User)
private ArrayList<Cliente> values;

public ExampleSpinAdapter(Context context, int textViewResourceId,
        ArrayList<Cliente> values) {
    super(context, textViewResourceId, values);
    this.context = context;
    this.values = values;
}

public int getCount(){
   return values.size();
}

public Estado getItem(int position){
   return values.get(position);
}

public long getItemId(int position){
   return position;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    TextView label = new TextView(context);
    label.setTextColor(Color.BLACK);
    label.setText(values.get(position).getNome());
    return label;
}

@Override
public View getDropDownView(int position, View convertView,
        ViewGroup parent) {
    TextView label = new TextView(context);
    label.setTextColor(Color.BLACK);
    label.setText(values.get(position).getNome());
    label.setPadding(10,10,10,10);
    return label;
}
}

Instantiate the adapter by passing your list of clients

 ExampleSpinAdapter  exampleSpinAdapter = new ExampleSpinAdapter(getActivity(),
                        android.R.layout.simple_spinner_dropdown_item,
                        clientes/*Sua lista de clientes*/);

spinner.setAdapter(exampleSpinAdapter);

If you prefer, you can create ArrayList and popular as the names of the clients and then pass them by parameter in the ExampleSpinAdapter, and make small changes in the Adapter