Preferred method of overriding an emacs lisp function?

There's a "new" advice method which can set up function replacements even before the original one is loaded.

(defun something-fixed ()
  ...
  )
(advice-add 'original-function :override #'something-fixed)

This even works in combination with eval-buffer of e.g. your init file.

@iainH You tend to get to a useful answer faster by describing what goal you're trying to accomplish, then what you have so far. I asked for code to try to help you do what you want to do without having to overwrite anything.

I still don't understand why you don't just use defun though? I suspect what may happen is you use defun in your init file, but the original function isn't loaded yet (see autoload). Some time later, you do something to cause the file with the original definition to be loaded and your custom function is overwritten by the original.

If this is the problem, you have three options (let's say you want to overwrite telnet-initial-filter from "telnet.el"):

1. Load the file yourself before you define your own version.

   (require 'telnet)
   (defun telnet-initial-filter (proc string)
     ...)
   

2. Direct Emacs to load your function only after the file loads with the eval-after-load mechanism.

   (eval-after-load "telnet"
     '(defun telnet-initial-filter (proc string)
        ...))
   

3. Use advice.

Of these, 2 is best. 1 is also okay, but you have to load a file you may never use in a session. 3 is the worst. Anything involving defadvice should be left as a last resort option.

Just to keep this question self-contained, here is how to do it with advice.

(defadvice telnet-initial-filter (around my-telnet-initial-filter-stuff act)
  "Things to do when running `telnet-initial-filter'."
  (message "Before")
  ad-do-it
  (message "After") )

This is obviously useful primarily if you want to wrap, not replace, an existing function (just redefine it in that case).

For much more on using advice, see the manual.