Stop altering letters (4)
I believe the answer is:
post
'letters' is the definition.
(thesaurus)
'stop altering' is the wordplay.
'altering' is an anagram indicator.
'stop' with letters rearranged gives 'POST'.
(Other definitions for post that I've seen before include "Job; upright timber" , "Upright - letters" , "Put a letter in the box - stop it" , "a placement: it's > part of one's goal" , "Job - letters - stake in ground" .)