Escaping characters in Emacs org-mode

With a current Emacs and org-mode you might try

* \under{}TARGET\under{}

If that is not automagically displayed as * _TARGET_ just try C-c C-x \, that should toggle the display of those codings between coding characters and coded character.

(In principle the same as I explained here.)

The code and verbatim markup will render the text inside as-is, without interpretation. Therefore, =_TARGET_= will probably work as you intend (you'll also get a different monospace typeface for that word).