We can define a type by combining multiple types like this to achieve required

type Stuff = { first: () => string } & (() => string);

const stuff: Stuff = useCallback(() => {
...    
})();

  stuff.first();

  stuff();

If you are interested in function static property typing you can use this example:

import { useCallback } from 'react'

interface Major {
  (): string
  first: () => string
}

const stuff = useCallback((): Major => {
  function first() {
    return "firstStaff";
  }
  function major() {
    return "majorStaff";
  }

  major.first = first;

  return major;
}, [])();

Playground

Please see this question/answer if you want to know more about typing static function properties

useCallback need an array of dependency as the 2nd argument then it can be self infer

import { useCallback } from "react";

export default function App() {
  const stuff = useCallback(() => {
    function first() {
      return "firstStaff";
    }
    function major() {
      return "majorStaff";
    }

    major.first = first;

    return major;
  }, []);

  return (
  ...
  );
}

Working Example: